Metallica is one of those bands where, even if the songs aren't good, you still won't regret having listened. Even if the song doesn't stand out in your memory, you enjoy having heard it--it's average quality if not better. Excellant background music if I'm working on something, and a good band overall.

Probably my favorite Metallica song would be I Disappear (or Enter Sandman). The only outright bad Metallica song I've ever heard was St. Anger. (But hey, every band produces a dud now and then.)