Innerspring mattresses remain the most popular type of mattress. They are generally less expensive to manufacture than a high quality HR or latex foam and they have been developed over time to provide a wide range of characteristics which provide different benefits for different needs. Although there is a wide variety of different innerspring designs, they can be roughly grouped into 4 main categories. These are Bonnell, Offset, Continuous coils, and Pocket coils. There are also many different names for each type but they will all generally fall in one of these categories.
Each of these has a different type of progressive response to pressure and some are better than others at providing softness with initial compression and firmness with deeper compression. There is also a wide difference between categories in how well they shape themselves to an individual body profile and help the comfort layers form a deeper cradle to relieve pressure when necessary. While none of them are as effective at this as many foams or other materials, which is why they are only suitable as a support layer in a mattress, Pocket coils followed by Offset coils are the most effective at "body contouring" among the innerspring categories and make good choices when this ability is needed in certain mattress constructions that use thinner comfort layers. Bonnell coils are a good budget alternative when this quality is needed in a budget mattress. All innersprings can be made to be firm.
The thickness of innerspring wire ranges from the thickest 12.5 gauge to gauges above 16 which are much thinner and less firm although higher gauge thinner coils can shape themselves to a body profile more effectively. While the gauge of innerspring wire and the number of coils in the innerspring are the most commonly used "statistics" of innersprings used to measure its qualities, these by themselves can be very misleading.
All innersprings can be made less or more firm through the use of thicker gauge wire, a greater number of coils, a larger number of "turns" in each coil, tempering, type of steel used, different construction methods, coil shape, coil height, and different insulator layers that are placed on top of them. Bonnell, continuous coils, and offset coils are also joined together with a tightly coiled thinner wire called a helical and different types and designs of these can also make a big difference in how each individual coil is affected its neighbors and the firmness and conforming ability of an innerspring. Pocket coils are kept together through the use of fabric "pockets" which are joined together (rather than the coils themselves being joined) or in the case of higher quality versions by "hand tying" each coil to other coils near it. These pocketed or hand tied coils have more independent movement which gives Pocket coils their greater "shape conforming" ability while the other 3 types act more together in a group with other coils because of the helical connections. All other factors being equal, this gives them greater firmness.
Rather than get caught up in the many complex differences between different innersprings, which too often ends up becoming a meaningless distraction in the search for a better mattress, it is important to remember that the two most important qualities of an innerspring ... once you get past all the complexities ... are their ability to conform to your body shape if you are using thinner comfort layers in more progressive, layering constructions and their ability to hold up the heavier parts of your body so they don't sink out of alignment while you sleep. More than this and a general idea of which type of coil is best at each function is not really necessary.
Many mattress sales people ... and consumers ... reduce the whole experience of mattress shopping to an excercise in "coil counting" sometimes with the coil gauge "sprinkled" in, which is among the worst possible ways to buy a mattress. Many cheaper mattresses use higher coil counts using lower quality coils as an enticement to buy a lower quality mattress and increase profits. Since innersprings can all be made more or less firm and more or less conforming in many ways, and since coil count is only one of many ways to change how an innerspring performs, remembering the two basic functions of an innerspring is the most important part of the innerspring puzzle. In general terms again, if a queen size mattress has approximately 400 coils or more in a reasonable gauge of wire (around 14 or better), then you can expect that it will provide you with good support. Higher coil counts are really only necessary (and other factors are involved here as well) when the innerspring needs to be more conforming to help with the pressure relief of a thinner comfort layer.
Good quality Pocket coils in general have the greatest "response range" which means that they can be softer on initial compression and then firmer on deeper compression. They also have the advantage of providing greater "point elasticity" which is the ability to form itself around your body profile. This means that they are particularly suitable for use in "progressive" mattress designs or with comfort layers like natural fibers which have less elasticity and ability to form a pressure relieving cradle as they can "borrow" this ability to some degree from the innerspring beneath it. In their best quality they are the most expensive innerspring to manufacture however there are many cheaper versions that are being imported. How they are joined together, the nesting pattern, and what surrounds them is important to quality as the biggest disadvantage of a lower quality pocket coil is that they can migrate with use through lower quality pockets, fabrics, or methods of connecting them together. North American made pocket coils are usually (although certainly not always) higher in quality than Asian imports. In the case of pocket coils ... more is generally better as more coils allows the use of softer higher gauge wire and gives the innerspring a greater ability to help the comfort layer in forming a cradle. If you are comparing here though (for those of you who just can't resist the temptation to count coils), only compare pocket coils to pocket coils since even rough comparisons between categories can be very misleading and in many cases meaningless.
Good quality Offset coils share some of the same qualities of Pocket coils and are often used in higher quality mattresses. They are an offshoot of Bonnell coils with a "hinged and shaped (squared) wire" on top which when joined with the coil next to it (with a helical wire) creates a hinging effect which flexes under softer pressure while the main body of the coil "kicks in" with deeper compression. There are several versions including free arm offset and double offset (has a squared off section on each side of the coil). Some of these hinges extend to one side only while others extend to both sides. Some are also hinged on the top and bottom. They have the ability to be both soft on top and firmer with deeper pressure and can be made firmer than Pocket coils because the individual coils are joined together with a "helical wire" and can "share the load" more than a pocket coil. For those who need some softness in an innerspring support layer and then firm support underneath this, an offset coil is a very good choice. They are generally the second most expensive form of innerspring although in their more complex designs they can approach the manufacturing expense of some pocket coils. These too would be very suitable for either a progressive or a differential mattress design.
Bonnell coils are the original mattress coil and are a development of wire coils used under seats in horse and buggy days. They have an hourglass shape and can be made with many different wire gauges to make them more or less firm. The reason for the hourglass shape is so the middle thinner part of the spring can compress with softer pressure and then the rest of the spring "engages" with greater pressure to provide support. This method is not quite as effective as either pocket coils or offset coils however bonnell coils are a good compromise and a very simple and effective design. Because they are less costly to manufacture and provide good value in comparison to benefits, they are very popular and a good choice in lower cost mattresses. Because they can be made in both firmer and softer versions, they can be used in either main type of mattress design as well but are more suitable in a firmer version as part of a differential design.
Continuous coils are made from one continuous wire made into rows with "simulated" coils in each row. These rows are then attached together with helicals. They are very simple and inexpensive to make because they do not have the expense of forming individual coils and then placing and connecting them inside the innerspring. Because they use a single wire and are "strongly joined" with their neighboring "coils", they can provide a very firm support system but have a far lesser ability to shape themselves to a body profile. They are most suitable in a "differential" construction of mattress with thicker comfort layers on top of the coil. In this construction method they can provide good value in a lower cost mattress because of their low cost of manufacture and the firmness of the innerspring.
Innersprings, like all support layers, are meant to control how deeply your heavier parts sink down into a mattress and help the comfort layers hold up your more recessed parts when necessary. Since pocket coils and offset coils have the greatest shape conforming abilities, they usually make the best choices in more progressive mattress constructions with thinner comfort layers above them. Bonnell are a lower cost budget alternative. In differential constructions with thicker layers above the innerspring, firmness of the support layers becomes the most important factor and any of the categories can make be an appropriate choice. Again in general terms, innersprings from most to least expensive to manufacture are Pocket coils, Offset coils, Bonnell coils, and Continuous coils and the price of a mattress should reflect this but there is also a great deal of overlap and there are lower cost and higher cost versions in each category. Perhaps the most reliable way to evaluate the "quality" of an innerspring is based on the weight of the steel but this is not a spec that is usually provided.
Innersprings in general, as long as they are reasonable quality, the steel has been tempered, and they have an appropriate degree of firmness, are among the longest lasting parts of a mattress and will outlast most polyurethane and memory foams. They will not however generally last as long as latex foam. Depending on type, they also have a roughly equivalent degree of resilience or "pushback" to firmer foams and tend to be a springier than latex or polyfoam for those who like that feel. If the mattress you are considering reflects the lower general cost of manufacturing innersprings over high quality specialty foams (in most but not all cases) and the cost also reflects the type of innerspring and comfort layers being used, then innerspring mattresses can represent very good performance and value.