We got ourselves a Simba Hybrid Pro last Christmas. When it arrived the box just didn't look big enough, but when you open it (open it on the bed or you'll never shift it) you just wont think itll turn into a mattress that's almost 1ft deep.
Admittedly our old mattress was crap, but this one is fantastic. Whilst its deep and supportive I was amazed how cool it stayed during the recent hot weather.