I've heard some really beautiful stuff via this website that I otherwise may never have had the chance to check out...rare group performances with members who are long dead, gigs that took place at the other end of the country or in countries I've yet to visit, one-off shows with unique guest performances, etc.
As per the ethics of downloading live bootlegs- I pour the majority of my meager income into seeing live music and purchasing (primarily jazz) records. Of the artists I've downloaded, I've seen almost every one of them live or bought one or more of their records- and as per those that I haven't, the bootleg recordings otherwise have some value as an educational resource (in terms of history or transcription)...I sleep easy knowing how much I've given to most of these guys already, and I don't have any problem with it.
Bob Berg, Mike Brecker, Hiram Bullock, Kenny Kirkland, Jaco Pastorius, Joe Zawinul, Jimmy Giuffre...all wonderful musicians who have passed on, whose work is documented (the first two very heavily) on this website, alongside recordings of a hundred or so living musicians. Check it out.