Great performances are available, but the sound quality of the recordings can be sketchy. I always listen to the recording first before committing to purchase the FLAC download. I also paid for the WVIP subscription which goes toward download purchases and you get good discount (usually about $4.20 for a concert in FLAC).
What hooked me were the Mahavishnu Orchestra concerts and psych-rock (Dead, Airplane), but I've also picked up some great jazz, blues and folk recordings (Stan Getz, Taj Mahal, John "Spider" Koerner, Weather Report). A lot from the old Newport Jazz Fests.
They also have free downloads from sessions at Daytrotter Studios, but these are only in mp3 - got a couple good ones with Vic Chesnutt and Rodriguez (Sugar Man).