Got the open beta client today and I'm loving the game so far, I wasn't really expecting it to be as fun as it is. Right now I'm pretty much sticking to the scout class (don't know the name, it's the default light class) and doing non-stop flag runs. Usually works out pretty well. I really don't see much point to the other classes beyond defense however, they don't have the speed to capture or persue enemy flag carriers, and I think speed is the most important asset in Tribes.
Can't wait to play it a bit more tomorrow!