In reply to karen2: Probably best taking advice from a back specialist, but in terms of comfort I really rate the Exped down mats.
Bought one for the mrs this winter (she tends to sleep on her side, so the usual thermarests are pretty uncomfortable) and it's amazingly comfortable. Think it's about 3 inches think, yet weighs/packs the same as my thermarest.
Bit of a faff inflating it (especially if it's -10 outside and blowing a gale and you just want to get warm!), but totally worth it if comfort is a priority).
Think I paid about £110.