The actual plot of the movie is all right, though it follows a pretty predictable path (right down to using "Mr. Blue Sky" in a Good Times Montage) of a loser (Mark) suddenly finding fame and fortune with very little effort, and thus also finding that it isn't all he thought it would be, especially when the girl he loves has no qualms about telling him the only thing keeping her from marrying him is that she wants her children to have better genetic material. It all works out in the end, of course, though not before Mark "creates" the idea of Heaven and God.
What I found interesting about the film was less the plot and more the idea of the world itself. Or rather, I was more amused imagining the kind of world it must be, how certain things must have happened, without any lying. For example, Mark works for a film company that creates lectures about various historical events, but since he works on events from the 1300s, he has very little to go on, and is, in fact, fired at the start of the film because, as his boss tells him, "Nothing new is going to be found out about the 1300s." Once Mark figures out how to lie, however, he writes a screenplay set in the 1300s and reads it to the entire office, and one bit that we get to hear involves ninjas teaming up with space aliens. Later, I was thinking about it, and I realized that while the space aliens bit could have been accepted by everyone listening simply because everyone takes Mark's word as truth (which is where most of the humor derives from), but it could also very well mean that in this world, aliens exist. And just little things like that.
It took a bit to get into the movie (the first fifteen minutes are kind of annoying), and I probably wouldn't watch it again, but for a free rental, it wasn't too bad.