I do a fair bit but it's no-where near as much fun as offroad. For me it's mileage and fitness. Once you get reasonably fit you can start chasing cars and really ramping it up on the flats, chase that maximum speed!
But the real buzz of road riding is seeing someone on a bike in front of you. When yuo're out mountain biking, you'll see you're catching someone, pass them, that's it. On a road bike, you'll see them, catch them, tag their back wheel for a bit to catch your breath, overtake, ride like the clappers for 5 miles, look over your shoulder to see them on your wheel, put more power down, struggle, they'll pass you, you'll tag them for five more miles.
Then the racing will really begin.
if it wasn't for other cyclists who are up for a race, road riding would be crap. (Crazy downhills at 50mph excepted.)