I've only done "real" camping a couple times (with more experienced campers).
The longest I was camping was in Voyaguers National Park on a 2 1/2 week canoe trip. I didn't know better, but was completely comfortable on a foam pad (plain, not with the cutouts and shapes)... of course I was younger then. It was during the summer so it wasn't cold.
I tried to get away without using a sleeping pad when car camping at the Grand Canyon. I think the night got down to around 50 degrees, and I had a nice warm sleeping bag that I almost froze in because of the ground convection. I should have paid $10 for a foam pad... this was in the summer, too.
I recently read a lot of reviews and picked up a Sea to Summit Ultralight insulated air sleeping pad. I bought it as a do-it-all, but primarily for sleeping in a house I was working on with no furniture, and I didn't trust the Wmt air beds to work for long from reading the reviews. I wanted to also have a small portable air mattress for traveling and emergencies (I also keep a folding camping chair in my trunk).
I've only tested the air pad to make sure I understood the setup. It was a little more plush than a foam mattress. I got the XL, but if I got a second, it would probably be the L. The I think the XL is DC'ed anyway. I'm 6' 205 lbs. I haven't tested it out on cold ground, but if I wasn't as concerned with space, I'd probably just go back to a quality foam pad. I also purchased a blow up pillow (fits inside the air pad's case), but don't remember if it was the same brand or not... it's in my emergency bag now and I don't feel like taking it out.