A typical MTB has a high gear of 44-11. It always surprises me that people complain about the gearing as being too small - I think that pedaling technique might be to blame. I can sprint that kind of gear up to about 30 MPH, which is a lot faster than I can maintain on level ground, especially with the big knobby tires, suspension and less aerodynamic riding position. In fact, I think that those factors would likely make it very difficult to get that fast in the first place. Changing to skinnier slicks would reduce the gear a bit, but it's still possible to hit high 20's, perhaps still even 30 MPH with a 44-11.

I commute on a touring bike or my "hybrid," which is actually an old Schwinn with flat handlebars and a 3-speed hub. I've also ridden my roommate's SS MTB. Even taking into account the low gear (~60-65 inches), that thing is S-L-O-W. It takes forever even to accelerate up to speed. I suspect that the wide knobby tires and front suspension fork are to blame.

An MTB and a road bike can both be flexible - my roommate is just as capable of hauling groceries on his Redline as I am on my LHT. My LHT is almost as happy jumping curbs and racing down bumpy dirt roads as his Redline (yes, with slicks!). He can obviously hit curbs harder and expect to just roll over them, thanks to his suspension, while I need to pop the front wheel up. If we were doing more off-road riding, then he might have the better bike, but both of us are primarily road-riding commuters and tourers (no, he doesn't tour on the SS). For mostly on-road riding, a touring or cyclocross bike is probably more flexible and versatile than the MTB, simply because it is more in its element and can move more efficiently. If you spend a lot more time off-road, you might be better off with an MTB. For commuting, I don't think that it's making a mistake either way. If you don't care about the loss of efficiency you get when riding a suspended MTB on road with knobbies, than there's no reason to ride anything else. If you do care, and want to go faster, you might look at something a little bit more road-oriented, like a hybrid, CX or touring bike, or sport road bike. If it can take a rack and fenders and is built to handle the punishment of daily commuting, you really can't go too wrong, as long as you're happy with it.

Route/Rig: I ride a 2006 Surly Long Haul Trucker (just replaced my old Miyata 210 touring bike), he rides a 1991 Redline Conquest - yes, it used to be a mountain bike - with a Rock Shox fork. We have a daily commute of about 2 miles RT, so... very short. Yes, the LHT is overkill for this distance, but I will be using this bike, like I used my Miyata, for everything, including car-free transportation all over the region, day trips, overnight trips and loaded tours. This is one reason that my roomate is using the Redline for the time being, instead of replacing his stolen hybrid right away. When we had an 8 mile RT commute, this would have been a bigger deal. The route itself is pretty low traffic with a couple stop signs, with a quarter to a third of the route on a college campus. In this case, our particular rides make little difference.