What a bomb. The only growth the character Rueben (Ben Stiller) experiences is that he gives up being obsessive about taking risks in life to hang out with Polly, the unstructured one, and live through weeks of excruciating episodes of irritable bowel syndrome.
It's also a kind of formulaic romance but with the characters only promising each other that they'll - hang out (live together). If one of the criteria for romantic genre is that the main characters sacrifice something for the other - then Along Came Polly doesn't go anywhere. Rueben gives up his immature attachments to risk and germs, but Polly doesn't give up anything. These characters use each other in non-transcendent ways.
It didn't make me laugh - and the actors are all good comedians - by themselves or with others. This ensemble hit it big the first weekend. As I write this there's an ad on TV saying the film is "hilarious." Maybe you had to be there.