Essentially, the film works on the level of awkward misunderstandings and gross-out humour. It's done very well; if the original American Pie worked for you, then so will American Reunion. Stiffler is still Stiffler, Jim is the same character he was in The Wedding (as is Michelle), Ostreicher is extrapolated into a sports journalist but is the same character, Kevin is the same character he's been in every movie, and the only character whose really been tweeked since their previous is Finch, and not too radically I must add. If you've enjoyed the ride of the previous American Pie films, then this is almost necessarily, methodically, a film designed for your light amusement.
The essence of American Reunion is the combination of gross-out humour, best exemplified by Stiffler's actions, and awkward situation comedy, best exemplified by the relationship between Jim and his father (specifically) or the world (generally). Now these two comedic methods, gross-out and awkward situational, is what all American Pie films have done really well when the franchise is at its best. American Reunion is a quickfire barrage of set pieces, alternating and sometimes often overlapping between gross-out humour and awkward situational comedy.
The themes aren't very deep, but they are somewhat relatable; the characters aren't very deep, but they are somewhat relatable; the challenges faced by our heroes aren't very deep, but they are somewhat relatable. Overall we're not given a very deep film, but that's OK, it serves as a sturdy vehicle for the gross and/or awkward set pieces, and generated laughs from the audience around me, and myself, at an efficiency not easily replicated. Very much worth a watch with friends, but there's nothing to be gained from a second viewing. It gets two stars for having a beginning, middle and end, and for setting out to do what it intended, and the extra half for it's laughter efficiency rate.