I purchased the Queen-sized Sealy Posturepedic - Guilmont mattress from Costco last month. This is the firmest mattress available at Costco; my boyfriend and I decided on this model as a compromise between my preference for futons and his preference for traditional mattresses. The Guilmont has no fluffy pillow top features - this is a no frills, firm mattress, which was exactly what we were looking for. We have been sleeping on it for almost a month now and here's what we found:

-As a back sleeper and a former futon owner, I'm quite happy with it. Soft mattresses always gave me lower back pain.
-As a side sleeper, my boyfriend would wake up some mornings with a slight numbness in his arms. This mattress may not be ideal for non-back sleepers. We plan on purchasing a memory foam mattress topper to mitigate this problem.

Other info, specifically on purchasing this mattress from Costco:
-I paid a bit more as a California customer, since CA and NV have flammability standards. This mattress costs $30 less in other states.
-After purchasing at the store (or online, I imagine), you have to call the designated delivery service and place the actual order. Delivery takes about 2 weeks (10-15 business days, I believe is what they say.)
- Costco has a great return policy - just keep the receipt. The problem is that you have to lug the darned set to your nearest store.
- Since this is Costco, you can't separate your purchase, meaning that even if you don't want it, you're getting the box spring. You can get rid of it, but you can't make a return without it.

All in all, I'm happy with this mattress. I didn't think I would be very satisfied with any mattress as a futon user for so many years, but this feels quite firm and sturdy. I will probably write another review after a few weeks to report how my side-sleeping boyfriend feels about this mattress with a memory foam topper.