There are a few funny bits sprinkled throughout, but the premise of the movie never goes anywhere particularly interesting, even with the potentially outrageous subplot regarding religion as an outgrowth of chicanery.
The simple truth is, any comedian could take Gervais' place in the film, and you probably wouldn't notice. For a comic who likes to put his fingerprints on everything he does, it feels like he's playing it safe.
It's a high-concept twist too far: in Liar Liar and Groundhog Day, the worlds themselves were believable; whereas here the world (explained via a cumbersome opening voiceover) is simply too far-fetched and therefore lacks resonance.
There's no doubting that this is one comic who can really act. But maybe Gervais should leave the directing to someone who knows how to tell funny stories on film, if such a talent can be found these days.