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Just go to Kmart and get a cheapo bike...it does the same job....
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AAACCCKK! I disagree wholeheartedly!
For the extra hundred or so dollars you'd spend to get a 'real' bike, you'll shave off about 10 pounds, take a major step up in the component group, and get assembly and adjustment by people who actually know what they're doing. On top of that, the bike will last much longer and ride oh-so-much better.
I've worked in bike shops on and off for years (gotta get the wholesale deals to avoid paying $2200 or more . . . ) and would recommend looking at the three to five hundred dollar offerings from the 'big three' - Trek, Giant, and Specialized - as well as Gary Fisher, Kona, and any other smaller brand that catches your interest. Test ride everything that you can and get the bike that feels best. They all have their differences, but in that price range they all equal out quite evenly. A good bike shop is there to find the right bike for you and your needs. After you've ridden a few at the shops, go to Wal-Mart and ride a N.E.X.T. You'll run from the store in terror.
Just be honest with the bike shop staff. If you'll only take if on a trail once per year, you might not be happiest with a mountain bike. Test ride, test ride, test ride!
Good luck and tell us what you decide on.