We definitely use bags, and pads most of the time. The only problem with hammocks is that since you'll be compressing your bag on your backside, it won't offer much, if any, insulation. Insulation requires loft. Sleeping pads for ground camping are actually used for this purpose as well, in addition to comfort. Typical pads like you use on the ground are pretty crappy in a hammock... you move around too much and they can slide out from underneath you. Hennessey makes an insulation system. I've tried it and didn't like it at all. Overpriced and difficult to get setup correctly. If you're serious about hammock camping, a hammock underquilt is the absolute best way to go. Just search "hammock underquilt" in google and you'll find a few suppliers. Perfect Trees and Jacks 'r Better are a couple that I've seen good reviews from.

I should also note that everyone is different with respect to what temperature they start to require the need for a pad in hammock. Most people seem to be okay above 70 degrees, but anywhere below that I definitely need one. Any bit of wind can even make 70-75 degrees seem cold, at least to me.