I'm very happy with the Blazing Needles 6" futon mattress.  The product is perfect for its price range.  There were a lot of loose threads on the seams, but otherwise it's great.  Expect there to be a very strong smell for a while after you take the mattress out of the package.  I've kept the window open a crack next to the mattress and the smell seems to be going away slowly!

Pros: good value, comfortable for its thickness
Cons: a little shabby around the stitching, but fixable