I encourage everyone to ride a mountain bike if you plan on bike on a road with a lot of traffic and potholes!
I read the thread of replies about where you live. Anywhere with that many potholes--and/or if you like to take off-road excursions on a whim--it sounds like a mountain bike is just great for you!
On the other hand, for anyone who rides on roads with a lot of traffic (or not) with relatively good road surfaces, I would strongly discourage them from riding a mountain bike. Get a bike that's designed for what you're doing.
Mountain bikes are designed for rough- and off-road conditions. They have fat, knobby tires and low gearing, and
- 1. Fat, low-pressure tires make for a much harder (more difficult) ride--if you're in it purely for the workout, that's great. Otherwise, you're liable to get discouraged and think biking's not for you because it just takes too much effort to get to work and back. (on the other hand, it may result in a more comfortable ride if you don't care about effort).
- 2. As mentioned elsewhere, knobby tires provide less traction (and probably more rolling friction) on normal road surfaces--more dangerous when you need to brake in a hurry, as you often do in [sub]urban environments.
- 3. Low gearing means your legs have to spin around a lot faster to go the same speed as a comparable road bike.
I'm not saying the average rider needs a race bike. Au contraire... if you're riding as a commuter or just "around town", getting a (generally expensive) road bike is just as naïve as getting a mountain bike that you'll never take off-road. For the average commuter in relatively flat [sub]urban environment, I recommend a 3- to 8-speed bike (gearing in the rear hub; eliminates all the complication and maintenance of a derailleur setup) with relatively narrow tires... and a comfortable seat.
Myself, my normal commuter bike is an old (~1980) Viscount, single-speed (not a fixie, thanks!) with high-gearing (52/16) because I like to go relatively fast. When I'm riding with my family, I ride an old Schwinn 10-speed.