I use phpBB on my own forums, but they're mostly smaller. One is kinda big, but probably a little less active than Chiptalk.
I haven't really used vBulletin, but it does seem to share a lot of the same look as phpBB forums generally do. I had to do a quick check to make sure that it wasn't the other forum software that I was thinking aout that I hate (ubb). I really didn't like that one at all.
Personally, I'll probably install it on my own server just to mess around with and see what it's like

I think phpBB does get a lot of security issues mainly because it's free and pretty simple to set up, and quite often the maintainers don't keep it up to date. Which is pretty hard to do when you have a ton of customizations, which I do, so I'm sure I have an idea of what a pain it is for you to keep it up to date
The only thing that slightly bothers me about the forums now (which isn't even related to the forum software) is the alignment of the menu in Firefox. It just doesn't seem right.
Oh, and the recent database connection problems here have been annoying as well
