Part of their popularity is due to their cross over appeal. Rock fans like 'em, prog fans, metal, stoners, jam band fans, indie fans. They are sort of like My Morning Jacket (not musically), but appeal wise. Some folks say MMJ is southern rock, but they appeal to a lot of people who would normally not like southern rock.
And the jam band scene, when they find a band, they are DEVOTED. Someone mentioned fans going from show to show, typical jam kids.
I saw them at Bonnaroo, thought they were horrible, had to leave the area because of the....ear splitting noize. I don't think the word melody is in their vocabularly.
But, lots of people really loved them. The place was packed and they ended up being the buzz band of the festival. I know a guy who loves everyone from Joanna Newsome to Death Cab to Zeppelin and he is a HUGE fan.
Weird.