I recommend the GT Aggressor 3.0. It's listed on GT's website for $370. I recently saw it at a bike shop for only $320. I've owned one for nine years and have put well over 5,000 miles on it. I mostly use it for urban transportation, but I also got some good use out of it riding on dirt trails. I love it.
Don't let someone talk you into buying a mountain bike with disc brakes. Disc brakes are more expensive and unnecessary unless you'll be doing some serious mountain biking through muddy and wet conditions. I can stop on a dime with the Aggressor, which has standard V-brakes.
If you have any specific questions about the Aggressor I'd be happy to answer them. Best of luck.
Last edited by GeraldF; 04-19-13 at 10:40 AM.