My original futon mattress from the year 2000 was the most comfortable ever.  Everything is made cheap these days, but this is decent.  It is thick enough to not feel the metal bars.  I got the 10" thick one and would not recommend anything less.

The factory/plastic smell does go away after a couple of days.  Punch it like a punching bag to fluff it up.  At first, it was really flat and I was worried, but I was able to fluff it up.

Based on other futon mattresses I've seen, I would recommend this one, again, at 10" thick.