What Happens in Vegas is designed as a crowd pleaser. Fortunately, it's done well enough that it can stand the passage of time. It was an instant hit, and I must say, it's easy to watch over and over again.
Cameron Diaz who plays a modern power woman and Ashton Kutcher who plays a lazy party guy are funny as hell, and perfect for each other. Their sidekicks carry the brunt of the crazy behaviors and make the main characters look more normal.
The scenario is a nice roller coaster with few pause that only serve as jumping board for more wacky entertainment. The romance itself and the sweetening of the story is even well done. The souring was also handled pretty well, they didn't go off the deep end, and did it simply. The soundtrack is upbeat and current, and the editing hide some nice surprises.
The only thing I could reproach is the plainness of the dialogs. Nothing spectacular there, but no clumsy attempts either and it's funny, so it's difficult to really complain.
Weirdly enough, no sex, which is good, it would have just ruined the vibe. It's a good chick flick, as well as a guy flick. There's stuff of everyone in their.
For good entertainment, see it.