Hi GaryA,
I don’t have any personal experience with their mattresses but you can see some comments about Sleep Science in post #2 here.
There is more about the Sleep Science 11" Black Diamond in post #4 here and a forum search on Sleep Science Black Diamond 11 (you can just click the link) will bring up more comments as well.
There is also more about buying a mattress from Costco in post #4 here.
Each person’s experience on a mattress can be very different from someone else and what one person finds too firm another will find too soft but the good news with Costco is that they have a good return policy so each person can decide for themselves based on their own personal experience. I think if anything though that most people would probably put this mattress in the firmer side of the range than the softer side.
I would also keep in mind that if you do decide to purchase this mattress that you can buy it as a mattress only and then you could choose an adjustable bed as a separate purchase if there was a different adjustable bed that provided better features in the same budget range. Post #3 here and the adjustable bed thread it links to may help you decide on which adjustable bed would be best for you. Of course if you compare different adjustable beds and the Sleep Science is still the best option for you then you can still purchase them together but I would always compare mattress to mattress and adjustable bed to adjustable bed and treat them as two independent purchases so you are confident that each one is the best choice for you without being “forced” to buy them together.
Most furniture stores would carry wooden bedframes and if they are a standard size (and you can measure to make sure) then the adjustable bed should fit inside it.
Just in case you haven’t read it yet … I would also make sure you’ve read the mattress shopping tutorial here which has all the basic information, steps, and guidelines that can help you make the best possible choice.
Phoenix