In general, I advocate trying a mattress before you buy it. Everyone has their own preferences and foibles, and buying the surface on which you’ll spend 1/3 of your time without at least a cursory lie-down seems a little reckless. On the other hand, sometimes good reviews and a great deal (and a spare bedroom, or an un-picky kid) will cause me to temper that advice.
Like now—today Amazon has this Zinus 12 Inch Gel-Infused Green Tea Memory Foam Mattress (no, I don’t know why you would want a mattress that has green tea; wait, I lied, the green tea is used as an odor inhibitor, apparently) available at a deep discount if you’re looking for either a twin or full. The twin is down to $139 (23% off) and the full is down to $159 (36% off and its lowest price ever).
It’s averaging 4.5 stars with over 1,000 reviews, so if I needed a mattress, I’d probably chance it. That said, do make sure you actually like memory foam if you’re considering it, because it’s a very different experience than sleeping on a traditional innerspring.