I have no idea why critics slammed it, I just watched it and thought it was really good. A nice urban fairytale. You're probably right that Sandler in the lead role led people to expect another silly comedy like Grown Ups or Jack & Jill which he did just prior, when actually this was a serious dramatic role for him. If you notice, all the comedy is delivered by his alternate personalities (other actors playing him) while Sandler's scenes were all pretty serious, low key stuff. I think he really gave the character depth as the quietly tortured loner who's full of sadness and bitterness but he still has a good heart. It's a very complex role, especially within the context of a 'comedy'.