Justin and I love our Thermarests. As we haven't done the backpacking thing yet (see "The Next Step" thread!
), we both have the full-length, self-inflating (not foam cell) model. Bought them after one too many sleepless nights on rocky or hard-packed ground, the latter being a frequent problem when state park camping and thus confined to the designated tent pad at one's campsite.
Ours have a somewhat course feel to them, which is enough to keep our sleeping bags from sliding off or down them too much. At Hill Country SNA in May, I ditched my sleeping bag and slept only on my Thermarest, due to the heat and humidity; I slept comfortably, with a stuffsack wrapped in a t-shirt as my pillow.
At a short 5'5", I'll consider getting a 3/4 length Thermarest in the future, to lighten the load... since we're always carrying camera gear, as well as camping and hiking gear.