Once they quit farting around and play, they are rather tight.
But for all that everyone seems to love Grateful Dead's live work, I am much more a fan of their studio recordings. There's a precision of harmony they got in those controlled conditions. And I don't think that it couldn't be replicated live on stage, I think they wanted something else for their concerts. Someone has described a Grateful Dead concert as a "group meditation with the audience." That may have been a peak experience if you were there, but after-the-fact, it simply does not sound as good. Compare: