Basically, Julianne Moore slept with Kevin Bacon and now wants a divorce from Steve Carell. He is, naturally, rather depressed about this, and starts spending a bunch of time at the bar where Ryan Gosling goes to pick up women. Sick of hearing Carell whine about his wife, he decides he's going to help him fix his life. Cue a makeover and a master class in picking up women. I know it probably sounds dire, but it's very entertaining and then...well, I don't want to tell you the rest and spoil it for you.
I was pleasantly surprised, though. It was much better than I thought it would be. It was funny and at times genuinely moving (I saw it late at night, so exhaustion may have played a part with the latter) and pretty much anything you could want from a film. There's even a sort-of car chase and sort-of fight scene.
If I had one complaint, it would be that it's a tiny bit long, but I'm not sure what could be cut in order to fix that. And there was a part when Carell wonders why Gosling is helping him sort his life out, and it occurred to me that he'd been in the same position and that someone had helped him through it, and he was now paying it forward, but that didn't happen. And I was glad. It was too obvious, too cliche, and the film actually manages not to stray into cliche territory.
So, to sum up, go and see it. It's really good, and if you weren't that impressed by the trailer and absolutely loathe Steve Carell (like me) you should try to see it anyway. It's worth it.