I saw it the other night and quite enjoyed it, though I'm definitely glad I saw it in the theatre with an audience because I don't think it would have been as funny at home.
It's a bit fat, there are scenes that definitely overstayed their welcome (the Beatles scene, which from the trailer looked like it would be really funny, ended up being long and kind of tedious), and overall it could have benefited from some liberal editing. With that many cameos and bit parts though, most of them likely friends of Apatow, it must have been very hard to cut things out, but unfortunately it feels long.
The performances are fantastic throughout, and really make the movie what it is - the script is decent but a bit shallow, for the most part merely stating the cliches of the biopic genre in a heavy-handed, obvious way, but the actors are so good that they make even the plainest lines funny. If a crop of less-talented actors were given this script I don't think it would have worked as well. John C Reilly is really great and does surprisingly good imitations of Cash and Orbison (Dewey Cox is a blend of those two) and even Bob Dylan.
Overall I got some solid laughs out of it and it's worth seeing.