My own opinion is that the first two-thirds are well above average for a video game movie adaptation (i.e., its not bad), and the last third just completely falls to pieces as regards pacing, plot, and resolution.
I particularly found it stupid that we get a ton of dramatic build-up to the scene where Dastan finally reveals the truth to his oldest brother, only to immediately have all of that mean absolutely nothing when the dude is capped within 30 seconds. It was bad enough with his middle brother, but at least there it actually meant something (what with his brother spending his dying breath saving Dastan from the Hashashin), while his oldest brother might as well have been vacationing in Tijuana for all the relevance he had to the storyline in that scene.
As far as the end being a gigantic push of the reset button -- I can't (and won't) complain about the History Eraser Button being pushed when the entire movie's plot is based around a time-rewinding macguffin, but I can complain that it was handled clumsily.
And my last gripe is that holy damn, was there a total lack of chemistry between Dastan and the princess or what? Couldn't they have cast someone else for her? Gyllenhaal actually did far better than I expected him to, quite well in fact, but his efforts were largely wasted when they handcuffed him to a very unsympathetic and poorly acted female lead.