You can get a better used bike for $350. I would maybe consider getting a cheaper $100-150 in good shape steel rigid frame MTB to see if you like to MTB. If you like MTB then save up some money, and buy a decent hardtail (or a fully if you think you need one..but add some time to saving up =P). A hardtail with good front suspension fork/wheels/brakes and line choice is more than enough bike for A LOT of riding. This also depends on how smooth you like your ride I guess.

Edit: Remember it is about sport and fun. If you aren't having fun (which you probably wouldn't with a crap bike that keeps falling apart) you might as well just be jogging or lifting weights.