I liked it, but... *MINOR SPOILERS*
There were some things that were disappointing to me.
First, their use of swear words. I think they used them maybe 6 or 7 times the whole movie. There are tons of choices, but every single one (except at the very end, where they didn't let the full phrase get out) it was taking the name of the Lord in vain. Most people won't care about this, but I did. They easily could have used other words in place of those, but didn't.
There are a few parts of the story they should have expanded on. First, we never really find out WHY things changed. I would have liked an explanation. I mean, the power went out, and that's when it happened. But what exactly caused it? They probably figured it would distract too much from his music story line and all that to get into it, but I want an explanation dammit.
I also thought it would have been nice if we saw more things that had changed. We saw a few, but mostly only for the comedic effect of him looking it up in a search to get something different as the main search hit, and a few quick scenes. It would have been nice to see the impact of the changes more than we did. He also doesn't "look" for other changes (except quick searches of other well-known bands), just kind of stumbles upon them. I'd be digging around for that crap.
Third, I thought they should have had more about the others who remember the music as well. I think they kind of threw them in to show him that he wasn't crazy and to ease his conscience for awhile (and for that whole Lennon bit), but other than them being in a few scenes freaking him out, and them dancing at the end it was kind of a mediocre part of the story. And how did they know they were the only 3 in the whole world who remembered? Billions of people in the world and the only three managed to find each other?
Overall it was a fun movie, better than a lot of the crap they're putting out these days, but timewise seemed long for the story we got out of it. I also have to admit I liked how they chose to do the ending. I was totally expecting something different, and would have been disappointed if they chose the typical route of it being a dream sequence or hallucinations while in a coma or something.