I would be very carefull about taking someone else's opinion on this topic. Years ago, after reading loads of positive comments, I bought a self-inflating mattress pad. I found it borderline useless, little better than a plain blanket. I, like some others mentioned above, now use a twin-size air mattress. It packs better, weights about the same, and has never lost air pressure during the night. It is at least as comfortable as my regular bed. When posting these comments on a different forum, I was supported by some, and lambasted by others. It appears that what is comfortable to sleep on is a very personal, individual choice.
Second point: If you need a pad under your sleeping bag to keep warm in typical summer camping weather, you probably have a cheap, Walmart type of sleeping bag. I paid about $100 for my sleeping bag as a Cabelas clearance item. I have slept in weather as cold as 40F and I have yet to sleep through the night without opening the bag because I am too hot, and I am definately not a warm-blooded person. I would spend a bit of money on a decent sleeping bag and skip the extra pad.
Third point: A knit ski type of hat will do wonders for helping stay warm if your sleeping bag isn't the type that can cover your entire head.