I've used this for over a year on a large forum with over 20 million posts. It's been working with no problems. It's got some surprisingly useful features, like being able to pretty much force a user to actually see the warnings and to let them see ones they've received in the past.
Xon improves what is already there and if there is some function missing in the core, he adds it. You need his addons to make xenforo perfect. This is on of the must haves for us. Makes moderation much easier.