The best thing I can say about this film is that it is well-paced. It did not fall flat. The next best things are the supporting performances by the actor playing foppish groom-to-be Edward, the always marvelous Holland Taylor, and the actor playing Taylor's husband. The actor helping to critique Messing's potential outfits in an early scene brings a delightful absurd-yet-winning quality to the proceedings. Okay, that's about it on the positive side.
Dermot Mulroney, whom I adore, is far more believable expressing contempt for Messing than in any scene where he's attempting to be either businesslike or supporting. As others noted, he appears to have no emotional investment in this enterprise other than wrapping it up as quickly as possible. Messing, on the other hand, sincerely tries to carry the film and to create the illusion of chemistry with Mulroney. She is adequate doing so, but is simply not strong enough an actress to pull all of this off without help. Give her an "E" for effort and a "C+" for achievement. Given Mulroney's indifference, the one thing that could've helped Messing would have been a brilliantly cavalier Irons-esquire performance from the actor playing her ex-fiancé, Jeffrey. Instead, his performance is weak-kneed, mewling, and feckless.
If I tried to analyze this one any further, I'd obviously be paying more attention than most of the people involved. It's harmless enough to pass time if you have nothing else to do, and less obnoxious (and less creative) than The Wedding Singer, but you're much better off watching "My Best Friend's Wedding" again.