To enjoy this film, you have to not expect it to be anything other than what it is: a combination of pop-video, action flick and general celebration of the feminine charms of Cameron Diaz, Drew Barrymore and Lucy Liu.
It's not The Godfather, but who cares? I think you can tell that director McG had a lot of fun making the film (which shows in the excellent commentary by him and the Director of Photography), and his visual style is heavily influenced by his standing as pop video director. So much of the film is so lushly produced they can get away with quite a lot of silliness. For example, the scene when Diaz and Luke Wilson visit the set of Soul Train sees Diaz throwing down her thing on the dancefloor, and it's all so watchable. Who cares if it's a little dumb?
Likewise, there are a few fight scenes (obligatory since The Matrix raised the bar on hand-to-hand combat) which have genuinely been choreographed well, and it is the girls doing most of those stunts; again the pace keeps you involved.
I haven't gone through all the special features on the DVD, but the commentary was really interesting to hear - I wish more films had these!
So, you know ... the plot is a silly and far-fetched, but that's what films like this are all about - I'll take this over the twaddle that the Bond franchise has become any day.