Decent Carbon (read Easton) bars are great. They're stronger than ally, are lighter, and they do tend to damp a little bit of trail buzz.
Of course they are less "impact resistant" than ally though, and when they fail they fail in a big way.
Still, though I've broken many components on mountain bikes in the past, including bending a shimano XT crank arm, I've still not broken a set of Carbon bars!
I will still only trust Easton though, as I've heard of too many other manufacturers bars breaking. But that's just my prejudice…
Carbon bars do cost though, if you've got £100 burning a hole in your pocket to upgrade your bike with, there are many other ways you will get a lot more bang for your buck than buying a carbon bar.