I've only seen a couple of them. The AC~DC cover band was fantastic. The KISS Army, the "official" cover band sounded like them but, having seen the real thing, and met a couple of them, I had a hard time letting go of the physical differences.

I view tribute acts as fans of the band and it's just a way of sharing the music. Some are better than others of course and there really isn't anything like the real thing

Not very impressed by most of them, although I've seen a few that were pretty good.

All depends on the quality of the tribute band. I have a friend who's the guitarist in 4 tribute bands (AC/DC, Led Zeppelin, Santana, and Aerosmith). I've seen 2 of them and they are amazing. I've also gone to see tribute bands and was less than impressed.
Best tribute band I've ever seen- Australian Pink Floyd.

Well, you're never going to experience Jimi Hendrix live....
but Randy Hansen is a close rendition.