Just you wait, you'll get addicted one of these days. Oops, I suppose that "addicted" was a poor choice of words when talking about the Grateful Dead.
Anyway, despite the fact that none of the group have(had) good singing voices and they cannot(or did not) like to sing in sync, what do you have against them?
I must say that for live recordings, the sound of the quality of the music seems especially good. But, since I've just converted to being a Deadhead, I have not yet heard a studio album of theirs.
What I think that I like is the blues element in their songs. I don't care for pure blues, but when it is mixed with other genres, it makes for some excellent music (ever heard of Elvis Presley and his songs?).