A Really Good Film, For What It Is First of all, a little suggestion: If you're expecting "The Mask", or "Ace Ventura", then just move along. Nothing to see here. This isn't that kind of comedy. (It's arguable, in fact, that it's less a comedy than an idea movie, of sorts).
Carrey gives an admirable performance as a self-absorbed mediocrity in the beginning of the film. He's having a bad day, and towards the end of it, you start to wonder what Anniston's character ("Grace", fittingly enough) sees in him. At that point in the movie, it's pretty difficult to watch, because you don't have any real sympathy. So Bruce gets a page (on his pager) from God, and after the requisite (for this kind of movie) skepticism, takes on God's job for a week.
There are obvious problems with the premise. If someone ever really did acquire the omniscience, omnipotence, and omnipresence of God which is presumably the heart of His job, that person would be able to handle the tasks put before Bruce. It's one of the movie's conceits (essentially mandated by the Christian philosophy undergirding the movie) that Bruce is given God's responsibilities without being given his omnipotence. If Bruce were given true omnipotence (let alone omniscience), there wouldn't be much of a movie, so that is a necessary evil.
In my mind, this movie isn't so much a comedy as it is a love story -- albeit mostly an unrequited love story. Grace, Bruce's long-suffering girlfriend, obviously loves Bruce, and is there for him. The cliché breakup and their even-more-cliché reconciliation aren't really the climactic points of their relationship, within the movie. That climax happens when Bruce listens to Grace's prayers after their breakup, and in how he reacts to them (which I won't spoil for anyone). I've seen the movie twice so far, and that scene towards the end of the film always makes me tear up.
In short, the comedy in this movie isn't great, and Anniston herself isn't given much to do. (A significant amount of her character's depth comes from what Bruce sees or hears *about* her, rather than *from* her.) The physical comedy is good, but not great. (i.e., it's not "The Mask") The thing that, to me, makes the movie worth watching is Bruce's initial fun with his powers, watching the consequences of that fun, and his eventual epiphany at the end concerning what it really means to love Grace.
Not a perfect film, but certainly worth watching.