Playing the same three chords for 15 minutes while trading solos isn't really what jam bands are. I mean, like any "genre," there are good bands and bad bands, so maybe some bad jam bands fall into that category, but in my mind, the best jam bands are the ones that have a sense of adventure where the members start out with a song, go into a jam, listen to each other, and it organically evolves into something completely different, something that the band members themselves (much less the audience) didn't expect.
That's the great thing about going to a jam band show: you have no idea what to expect. All you know is that it's going to be something you've never heard before, it's not going to be the same show they played the night before, and (assuming it's a good band) it's going to be good.
That said, there are definitely some really mediocre and boring jam bands out there.
I've been seeing Phish since 1995, saw them this fall and it's incredible that they're still really bringing it all these years later. I love the Dead and got to see Jerry (his last show was my first), and really enjoyed Dead & Co. the two times that I've seen them (notwithstanding that I don't really like Mayer's voice, although I love his guitar work with Dead & Co). ABB, Galactic, Derek Trucks Band, and older Santana are also excellent.