Garage Band is fantastic and I highly recommend it's use for anyone of any level of ability. I use LOGIC 9 which is sort of like the big brother to Garage Band. Your track has great guitar tone, and the riff is great. One thing i would suggest is work at getting a little tighter with the drums, perhaps even rethinking your drum loop. In metal music, the lock between the rhythm guitar and drums is clutch. You are on your way!