I think Ricky Gervais can be a "love him or hate him" kind of comic. You have to enjoy the sort of deadpan style he uses, because like a lot of comics, he has one joke that he does in a lot of different ways. I've enjoyed him in smaller roles in other people's movies, like Stardust, but this was my first time seeing him carry a film. To that end, he doesn't do too bad. I can, however, blame him for the film's shortcomings, because he co-wrote and co-directed it.
The Invention of Lying is a pretty accurate title. In the film's universe, lying is not a part of our way of life. Everyone tells the truth, all the time. If you think someone's kid is ugly, you say so. If you go out with someone who you don't find attractive, you tell them up front. It seems like an efficient way of life. Then, one day, Mark Bellison (Gervais) is having just about the worst day of his life, and magically stumbles across the concept of not telling the truth. Except, in this world, "truth" as a concept doesn't exist any more than "lie" does, because there has never been another way of saying things. I say magically, because aside from the CGI dive into his central cortex where a few flashing lights go off, there is no explanation for why he can suddenly do this. Maybe stress triggers it. Anyway, in typical comedic fashion - Bruce Almighty did this idea pretty good - Mark takes off with his new-found power and decides to up his position in life a little.
He works as a screenwriter for a film company, and in this world, imagination doesn't exist (because it isn't the truth). Movies are just people sitting and reading a history book about a particular historic event, and that's what people go to see. Mark, having invented lying, writes a screenplay of outrageous proportions, but as far as everyone else knows, it's pure fact that just wasn't discovered yet. This leads him to wealth and fame, and the inevitable knowledge that happiness can't be bought.
There are some clever touches along the "only truth" way of life: commercials can't exaggerate or make outlandish claims, so they just sell you on the facts. Coke's slogan is "because we're famous", while Pepsi just goes for "When you don't have Coke".
The biggest problem with a premise like this - if X factor was changed about the world, how would it be? - is that holding it together while still forcing a story through it tends to be problematic. Usually after 20 minutes or so, the novelty wears off and we end up with another very tired story with very little to offer in the way of new ideas. This movie could be considered a success relatively speaking, as it probably takes closer to 45 minutes before it's mostly abandoned as a plot device. In fact, I'm fairly sure that the last half hour contains no lying whatsoever. There is a lengthy side story involving Mark more or less inventing Christianity, but it fizzles and is forgotten by the end.
Another inherent problem with this particular plot is that it practically forbids character development outside of the protagonist; nobody else understand lying or can make use of it, so there shouldn't be any real changes in how they live their own lives. For the most part, this remains true; Jennifer Garner's love interest, out of necessity, grows as a person, but it's pretty fuzzy as to why this happens. She reacts to various situations as though she hasn't been in them a dozen times before in the lie-less world of the movie, for no other reason than she's supposed to. It takes away from the concept on display.
The acting is fine, and Gervais actually shows some chops aside from just cracking a joke. There are several great cameos which I don't want to spoil, but the cop in particular was a great addition.
I was optimistic that the premise could float an entire hour and half movie, but the last act degenerates into cliches and was probably re-written to hardly resemble the original draft to make it a happier ending. This shouldn't be a romantic comedy, anyway - Jennifer Garner looks about 20 years younger than Ricky Gervais. What were they thinking there? If they had skipped the boring romance angle, I think this movie could have been a much better final product. Unfortunately, even British comics aren't immune to Hollywood producers. Not a bad rent, but don't expect much.