Do you mean for the Halloween parties specifically, or just to the parks in general? I'm surprised Sherry hasn't gotten in here yet, but there is a Halloween superthread that will also help answer your question beyond what I can do.
I only went for the Halloween parties on the 19th and 29th, neither of which sold out. According to friends, the Halloween night party did sell out, pretty early in the day. As you could go in the park 3 hours before closing and the subsequent start of the parties, of course that's what we did. They weren't all that busy.
I remember waiting in line for Matterhorn before the park closed, and it being kind of the regular 15 minute wait or so. We also did Space with a 5 minute line, but I'm pretty sure that was right at the start of the party, so you'd need to show you were attending the party to get on. On the 29th, we watched Celebrate! and found a spot for it easily by Small World. So, not crazy spring break/summer busy, but definitely do-able.
If you have the time/funds/inclination to attend the parties, I'd recommend it, because lines are short, usually not more than 20 minutes, if that, and there are more characters out and about that you wouldn't normally see.