I guess I'm crazy, but I saw it in San Francisco in February and "garbage" feels like hyperbole. I was at the Orpheum theater, it was packed, and everyone was applauding, cheering, and laughing from beginning to end -- the DeLorean's first appearance in the show, appearing out of utter blackness, was awesome, and the second half was much more finely tuned than the first half; when the DeLorean started "flying" at the end, everyone went nuts, for example.
A few songs definitely could've been cut (George's, Doc Brown's dreamers song, and Biff's, IMO) but I guess this is definitely one of those things where people have to just experience it for themselves or not, where one person calling it "garbage" can be countered by someone like me saying it was a fun time out. So for anyone reading through this thread, I'd just say, if you have the chance to see it, give it a shot rather than taking any one person's word for it -- if I'd seen this thread crapping all over it and decided to not see it, I would've missed out on a fun time celebrating a movie I love with people I care about surrounded by strangers who were also very much into the show.