Hi Squirrel,
You can read a little more about microcoils in this article. A forum search on microcoil will also bring up more information about them and many mattresses that use them. They are certainly a good quality, “safe”, breathable, and durable component. While the choice of materials and components in a mattress is always a personal preference … there are many people who have purchased a mattress that uses microcoils with a good quality layer of foam such as latex on top (including many who have purchased a Berkeley Ergonomics mattress) that are happy with their choice.
The most important part of the “value” of any mattress purchase is how well it matches your specific needs and preferences in terms of PPP (see post #13 here) but in terms of quality and durability none of the Berkeley Ergonomics mattresses have any weak links in their design or the quality of the materials.
Phoenix