I bought two of these for a twin bed and a daybed.  Have only had one guest sleep on it and they said it was very comfortable.   So, when I needed another twin, I purchased another one.   Firm but not too firm. 
I use them with a platform type foundations, no box springs needed which makes it easy.  I wish I had purchased one for the upper bunk but that called for a 6" but in hindsite, this 8" would have been perfect.