The Hangover's got a pisser of a trailer. http://www.apple.com/trailers/wb/thehangover/
The cast is sweet (Ed Helms is poised to explode one of these days...) and the director, Todd Philips, can be a very funny ringleader of R-rated comedy (see Old School or Road Trip for proof). However, his last effort, School for Scoundrels, was painfully bad. I have a feeling that The Hangover will be a return to form for him.
The early buzz on The Hangover is through the roof. A script for a sequel has already been commissioned. That says something. This isn't a special-effects extravaganza; it's a raunchy buddy comedy.
Also, of personal note in regards to Philips, I sat next to him at an opening night screening of Collateral in the Dome at the Arclight a few years back. Pretty funny.