Mountain Biking - handlebars

What is the performance difference between a FLAT handlebar and one with a RISE? I figure it was personal preference, but I was wondering if there were any benefits to them.

Most of it is personal preference. I use a riser, but used to use a flat. I prefer the riser bar out of the two. It gives the rider a slightly more comfortable upright position and eliminates the need for bar ends. I feel that it gives me better control than with my old flat bar (could just be that I've gotten used to the riser enough). It really comes down to personal preference. Some riders swear by a flat bar with a set of bar ends and others wouldn't switch from their risers if their life depended on it. Try to find a friend/LBS that has bikes with flat and riser bars. You can ride both (even if it's just quickly) and at least see if one setup just won't work for you at all.

-Moab

I really like my riser bars. It's only a 3/4 inch rise, but I think it's much more comfortable on the neck and shoulders. Sounds like a preference thing.

It used to be easy to get the proper handlebar height. Nowadays, due to things like the threadless headset, it's become much more difficult. Many bikes come with steerers cut too short for the average cyclist. Riser bars make it easier to get the proper height. Consequently, these turn of events have also limited the availability of flat bars with more sweep and width. You used to be able to get flat bars in a much greater variety of dimensions than you can now. Other people simply like the look of risers. I prefer flat bars with bar ends for more hand positions and I ride mainly XC.