I've had the Costco Casper Select Queen mattress for a little over a year -- and let me tell you, it's great! I generally sleep on my side and stomach, and have yet to have any complaints with it. In fact, my girlfriend ended up loving it so much that she got one for herself.

After doing a bit of research, it appears that the Costco Casper Select mattress, is essentially a renamed Casper Original -- a much more expensive mattress.

Other things to note: While Casper says that the zip-off top cover is not machine washable, I find the opposite to be the case. I have on more than one occasion washed the mattress top-cover. I used the Bulky Sheets/Tap Cold settings on the washing machine, and then tumble (no heat)/air dried the mattress cover, with almost no change to the feel of the mattress or cover.

Edit: as partytime71 has pointed out, this most likely voids the warranty, so do at your own risk.

Additionally, if you plan to buy this mattress, please use a box spring or at the very least a sturdy platform placed directly under the mattress. Slatted bed frames (without a box spring) will change the feel of the mattress -- make it feel softer and make you sink more into the bed. I know this, because my girlfriend does not have a box spring... While it still feels great, it isn't nearly as firm/sturdy feeling as it is when a box spring is underneath it.