My parents bought a sealy mattress for my old room after I moved out at Sams, its an ok mattress.  I'm not sure which model it is.  
I bought a Denver Mattress a while ago, and now that my body has adjusted to it, I sleep like the DEAD in it.  It's sooooo nice.  It wasn't cheap (~$350, but it's an extra long twin, which are a little higher) and it was the best model they had in extra-long twin.  My parents just bought a King size Denver Mattress pillowtop which apparently they love.  I don't know for sure how much they paid, I think around $1000-1200 for it.  
I'm happy with my Denver Mattress.  I didn't find the sales guy too pushy (he took my "no I don't need the special mattress treatment crap at $30 a bottle" the first time and didn't bug me about it) and I got a nice warranty with it.  The one my parents got had a small tear in it apparently, so they sent out a couple guys to get their old one and deliver a new one without too much hassle.
The problem with buying mattresses at sams is they don't usually have them out where you can try them out first.  Trying a mattress out first is key.  
You bring up a good point with the "non-sleeping bed related activities".  I think when I move into a bigger place and buy a bigger bed, I'll point blank the salesguy with "So how's the sex on this mattress?" 
