I've got a simba mattress which I got on one of their deals they have every other month.
The first 6 months or so it was alright. Now it's a bit ridgey. My side appears to have dipped a bit, the middle has a ridge then my wifes side has a dip. My daughter often sleeps in the middle between us!
The best mattress I've had was from loaf.com. I bought it when they were a relatively small business, I think they've shot up now though.