I'm using a portacot for day sleeps, as Oscar sleeps in my bed at night. I actually lay him on a lambskin.
It would be interesting to investigate if the bad back comes from THEN moving to a soft, springy mattress from a nice, firm surface. We sleep on a futon and our backs have never felt better upon waking since we got rid of the spring mattress about 2 years ago!
Anyway, that was just a typing out loud. But, yes, we use one for day sleeps because we can't justify spending money on a cot (or even if we borrowed one, ours is a TINY house!) when the longest he spends in bed is at night with us.