For me it pretty much depends on how good they are. I've heard some cover bands that are so bad you just wanna cringe. If they're really good though it's a good experience to hear stuff you'd normally never hear 'live'. I saw a Queen cover band do Bohemian Rhapsody and all I could think was 'what were these guys thinking??"
but I also saw a beatles cover band, who dressed like them and even looked like them, used the same vintage instruments etc... and they were excellent.
I think it depends on who's being covered too. Like if a band were to cover 'Rush' up here in Toronto it'd be like, what's the point. You can see em live anyways.