I'm pretty much in agreement with Brad. It really depends, and in most cases comes down to execution and the value (or perceived value) of the content behind the wall.
His Dribbble example is a great one. That buzz can be valuable, and if done well can create great momentum. However, if done poorly or for the wrong reasons it's just extra friction that can alienate people.
Users have to feel like signing up will be worth the extra effort, and to get a viral loop going, they have to feel like that effort has paid off once they're in. That's all about content and execution. If you've got quality content and a great execution, the hit to UX won't be noticeable. If you don't, or if it's not really "exclusive" then it'll reflect poorly.