Phish: saw them once in 1993. Good band, but it didn't do much for me and I've never seen them since. Strangely enough, however, I really like Trey Anastasio Band. The scene is more mellow and people seem to be there more for the music (as opposed to just being there for the party and the drugs.
Widespread Panic: saw them for the 1st time this year and really enjoyed the two shows I saw. I'll definitely go back next time they play the SF Bay Area. I found the scene at the shows to be a bit strange. People for the most part were very friendly, but there is definitely an "anti hippy / anti Grateful Dead contingent. Not really sure why, but it's definitely present.
SCI: I've heard their music and it did nothing for me.
As for some other bands mentioned in this thread...
Umphrey's McGee: I don't dig this band at all.
moe.: they intrigue me a little bit, but I've never seen them and don't know much about them.
RRE: I like what I've heard, but haven't seen them yet.
YMSB: I freakin' love 'em!!!
Greensky Bluegrass: ditto... love 'em.
Other great bands not mentioned...
Infamous Stringdusters, Mother Hips, Karl Denson's Tiny Universe, Stanton Moore Trio, Galactic, Anders Osborne.