They're a little bit lighter and, as Dan Burkhart said above, they don't look out of place on a road bike. I've got a pair on my one of my bikes and they're still rather easy to clip into (better IMO than the Look Deltas I used to have) and don't look bad, but I'm not sure if I'd get another set. They're not much lighter than two-sided SPD's and aren't as secure of a connection as a true road pedal.

I do use double-sided mountain SPD pedals (currently Xpedo MF3's but have used Shimano's in the past) off-road, and I'm pretty happy. The Xpedo's accidentally unclip a little too often, though, even with the tension pretty high.