I saw Wedding Crashers in February, but that's more of a romantic comedy than a true Frat Pack film. No such luck with Old School - this is pure Frat, and its plot (in which a group of almost-middle-aged men form a fraternity group) actually gave the Frat Pack its name.
Some people think that Will Ferrell is a comedy genius. I am not one of those people. Luke Wilson, like his brother Owen in Wedding Crashers, outshines the other leads. I didn't actually laugh at anything, though.
The film (by Todd Phillips) seems like Fight Club lite, with Vaughn as Brad Pitt and Wilson as Edward Norton. The director says this was a conscious decision, and the similarities between the two films are extensive. There's also a reference to The Graduate, when The Sound Of Silence is played as Ferrell falls into a swimming pool.
Old School is tamer than key frathouse films National Lampoon's Animal House and Porky's, though really there's not much point in making a tame frathouse film. (I saw the unrated version; the theatrical version is even tamer.)