I enjoyed it throughly. Sandler surprised me with the depth of his performance: behind all of Charlie Fineman's coping mechanisms, his drums and videos games and verbal jokes, there's a hidden well of pain. He's all surface, no memories, because he can't afford to plumb the depths. Cheadle, of course, is excellent as well: Alan Johnson is a family man who has no friends, no life outside his well-paying job and his wife and kids.
It's a quiet film, slow in places, but my attention never wavered. The slowly-rebuilt friendship between Alan and Charlie is very real, and even more than that I loved the ways in which Alan tries to help Charlie, and the ways in which they end up saving each other.
The PoV character for most of the film is Alan, and that worked exceedingly well. "Charlie-world" is as odd to him as it is to the audience, and as interesting, so we can follow along with him. That shifts, later, once Charlie takes the first step into confronting his pain. I didn't have a problem with that, necessarily, because it just takes the film from being Alan's story to being Alan and Charlie's story.
What I did have a slight issue with, and would have changed were I the writer, is the distance that crept in near the end. The events justified most of it, but it jarred a bit to have scenes where neither Alan nor Charlie were present. That could have been reworked, I think, and would have kept some of the emotional impact.
If you'd like to see a movie about friendship, about how much it adds to human life and how much we need it, then you should certainly see Reign Over Me. (whitemartyr, I think you'd like it; I wanted to call you and talk about it after I saw it.)