Surprisingly good.
The commercials for this looked terrible so I never bothered. But it was on cable so I gave it a shot. It's good!
It's a comedy. It's not Tree Of Life, it's not 2001, it's dirty jokes and destruction of property. If the leads had been guys (Seth Rogan as 'Timmy,' Bruce Willis as grandpa etc) it would have done better in theaters, but it wouldn't have bothered giving the characters any challenges or dimensions.
Of course Tammy is unpleasant in the beginning. No you probably wouldn't want to hang out with her. But I wouldn't want to hang out with most of Chris Farley's characters, or //anybody in an Adam Sandler movie. By the end I was on her side, and found her more sympathetic and interesting than any SNL also-ran who just spent 90 minutes getting kicked in the nuts and throwing food at rich people.