Most released covers I hear aren't very good, but then most covers are done at gigs etc rather than being released, so there's plenty I can't know about. Will just say that covers I enjoy include Feeling Good by Muse and Highway 61 Revisted by PJ Harvey. Awful ones that come to mind include All Saints' woefully misbegotten cover of Under The Bridge and that horrible version of Lady Marmalade with Pink, Lil Kim, Christina Aguilera etc a few years back.