The weight savings is nice but the stiffness is where it really comes in. I got to test the new Reynolds wheels over the past year and was blown away by how stiff they were. I actually rode 9 miles of Nat'l with 3 (4?) broken spokes on the rear wheel and it barely came out of true. Light with quick acceleration is what you want in a race wheel...
Per the articles at various websites already posted, (some) DH racers are going through a set of rims every couple of races. They claim to get a full season out of these ENVE carbon wheels. That would easily make them economically viable for their needs.
They must be hella strong, which seems to be backed up by Ken's experience.
"This is my favorite section...watch your step..."