I do both. I love mountain biking, but it is very inconveinient for me. I have to drive my bike 30 mins away to get to a decent trail, so its a weekend only thing for me. Its TONS of fun though. The weird thing to me is that I feel like the muscle and endurance I gain on my roadie doesnt at all transfer over to my MTB. I feel muscle soreness in totally different spots when riding my MTB.

I have a lot of respect for the real MTB'ers.. however, usually I just see guys on $3,000 downhill bikes riding around the trail at my local park. Their bike is so clean I dont think it has ever even seen dirt.

Anyway, my main reason for loving road biking is that I love the speed I can ride at. Unless I find some massive hill, and its smooth with no obstacles, I can not get the MTB up even close to the speeds I hit on my roadie. To me, that sensation of speed is pure adrenaline!