MORE had this discussion a while ago.
Here's the thread (I think you can read it as a non-member): http://www.more-mtb.org/forum/showth...iser#post83501
Here's the first response - full of generalizations and smarty remarks, and followed with responses that are exceptions to this. But so is the way of forum discussions!
"weight weenie racer boys use flat bars, bad boy aggro hucksters use riser bars. Me I use funky bars since I'm a singlespeed fixie hipster type.
It all comes down to how the bike fits you. To get the same hand position, you could get a high-rise stem and flat bar or a stem with less rise and use a riser bar. Either way you would achieve the same end result. Typically riser bars are wider, have more sweep and give you a more upright riding style which for bombing downhill is a good thing, not so much for all out sprinting on an XC course. There are more and more styles of bars coming out that give you more choice, I know Salsa has some flat bars with a lot of sweep (for a non-funky bar that is).
Some generalizations there, but gets the point across. How do you ride and what works for you, that is the most important thing. If a shop is pushing marketing hype over fit walk out and go somewhere else.
Just don't ever use barends on a riser bars, that's like wearing white after labor day!"
2007 Seven ID8 - Bontrager InForm
2003 Klein Palomino - Terry Firefly (?)
2010 Seven Cafe Racer - Bontrager InForm
2008 Cervelo P2C - Adamo Prologue Saddle