To balance all the outpourings of "I heart road riding" above, personally I find it depressing, demoralising, boring, dangerous (but not in a good way) and rewardless.
I have a feeling this is partly to do with riding out of Edinburgh, which involves shite roads, shite drivers, (generally) shite scenery, and shite weather.
Its just one long grinding slog to me. I hate it. But it does get you fitter, and beats riding an MTB for countless dull miles on road.