I'm a fairly serious mountain biker as well, and I'm wholly sold on carbon rims. In particular, I'm waiting on a set of rims to be delivered to rebuild some I9

I'm a fairly serious mountain biker as well, and I'm wholly sold on carbon rims. In particular, I'm waiting on a set of rims to be delivered to rebuild some I9 Hydra Trail S aluminum-rimmed wheels with carbon. I've been riding on carbon rims on both hard tail XC bikes and my full suspension trail bike for six years. I've also had aluminum rims on my single speed and hard tail trail bike. I can't count the number of times I've hit the carbon rim and had it be just fine, but all of my aluminum rims have some degree of denting from much lesser riding. Between the bikes with carbon wheels there's about 12,000 miles of trail riding and they've been just fine. After I dented the rim on the I9s and had a spoke break I decided a good winter project would be rebuilding the wheels to carbon rims because my previous experiences with them have been so good. I think the primary benefits of carbon are stronger sidewalls (less likely to dent) and that they cannot be bent and thus can't go out of true (unless the wheel is built wrong). Yes, they can be broken, but it takes a LOT more than it would for a comparable aluminum rim. (For reference, the rims I've beat on so hard are Light Bicycle RM29C07 and I'm replacing the stock rims on the I9 wheels with AM930.)