When I'm forced to go into my lesser-preferred LBS I always see they'll have two or three dept store bikes to every decent bike.
I started out MTBing on a 10-year-old Schwinn Frontier. The brakes were pathetic & it was heavy & cheap, but it lasted just fine for the first month or so until I got a 2001 GF Wahoo for $60. Granted I'm only 140#, but we still have the frame, fork, & components (stole the wheelset after I taco'd those on the Wahoo)