Had a mattress that was crapping out and giving me bad lower back pain. Tossed a Sensus on top of it and back pain went away. Sensus is definitely not as good now as it was new 2 years ago, but no pain yet. If you are getting back pain, either your foam is crapping out or the underlying mattress has gone bad.
I've also had some of the most comfortable sleep ever on a extra firm (but high quality) traditional mattress with no foam. Those mattresses are good for sleeping, but not good if you want to prop yourself up on one arm and read a book (which the foam is great for - you can go hours without having your elbow hurt or arm go numb).
If I was wealthy and owned a house, I'd purchase a high quality extra firm mattress. Foam is almost as comfy (with benefit mentioned above), cheaper, and more portable for when it's time to find another apt.