It's hardly new territory from the standpoint of the over-driven, uptight character who blames everyone else (including God) for his own misfortunes. In this case, God decides that it would do him some good to try being Him for a few days.
Unfortunately, for me, the first day or so was funny (mostly because Carrey carries it), but the rest of the movie slowly goes down hill. By the end, I was happy that it at least knew its limits and stopped at 1 hour 36 minutes.
This one isn't going to make it into this year's top ten for me, and I can't really recommend it with films like Bend It Like Beckham in wide release.
However, if you want a good God film, I'd suggest Oh, God! (George Burns, John Denver, 1977). The film is also a comedy, but it has a better message and is more consistently humerous.