Shawn: i would reccomend a downhill bike or a dirt jumper bike.
Most mountain bikes, though, are bought for fairly cheap, sometimes under $100, at department stores under brands such as Mongoose, Schwinn, or Magna. These "mountain bikes" are covered with fancy graphics, lots of gears, and suspension, but actually work very poorly:
1) The components are all bottom-of-the line, even if they have good names such as Shimano. They are heavy, poorly machined, and wear out or break quickly. The gears will usually grind and skip no matter how well you adjust them. Rims are often steel, which quickly rusts and bends out of round, is very heavy, and is never seen on decent wheels.
2) They are no fun to ride. Most of them weight at least 31-35 pounds, and the full-suspension models weigh around 45! Try riding up a hill on one of these. Most people just ride them around the streets, and the knobby tires soak up so much energy you can actually hear it. Full suspension will completely absorb any power you put out. It feels like riding through mud.
Mountain bikes are extremely popular, though, accounting for over 90% of all bikes on the average college campus. They are easier to ride than road bikes, but will prevent you from ever enjoying bike riding. If they are used for off-road use, they simply become a toy like dirt bikes that you can't use for everyday transportation. Overall, I think they are a backwards development in cycling technology.
"Dude-do yourself a service and buy a road bike like mine. Then see if you can keep up with me on the road."