I bought a generic memory foam mattress about 10 years ago and it's fine. I like it because it doesn't need turning and seems to be a hygienic material. Also, and I suppose this is the best recommendation, I don't give it a thought.
I gather this is the sort of thing that should be tried before bought. I don't think I'm that fussy though - for some reason I can sleep fine on a hard floor, and had a cheap futon for about 20 years.