I'll add to the list of "it's worth it" comments. It certainly is very cool.
As for selling out, I guess it depends. If you want to go through with others, then, yeah, it'll probably sell out pretty fast.
However, if you want are looking tickets for just yourself (or maybe you and one other), then I believe a lot of times they might have an opening or two. I know in our group last year we had someone just buy a ticket on the spot, and we still had one open slot. So if it is just you or you and one other, and are flexible about when to play, you *might* be able to get tickets late or even get a ticket there. However, I wouldn't rely on that entirely. The event keeps getting more popular every year.
If you are looking to go through with a whole group, then I bet you'll need to get tickets in the first couple days of registration.