I haven't had any experience with vBulletin beyond than a couple demos, but from that time alone, I can firmly express my dislike for it.
However, I've admin'd a relatively small IP.Board 2 board for someone for almost a year now, and I don't like it much either.
vBulletin gets on my nerves largely due to its interface. As stated above, the ACP is rudimentary and still very much resembles a phpBB2-era ACP. Beyond that, the posting interface feels tired and old. I'm using the Quick Reply to write this; look at how much space it takes up and yet still wastes. The text box width is about OK, because much wider and reading becomes a pain, but it's far too short. There's OS-styled buttons below it, but the button to open it was themed. The textbox I'm writing in fits in badly with even the default theme, looking like it was ripped straight out of Office 98.
On the other hand, vBulletin feels comparatively quick and stable compared to IP.B. Its interface may be buggy in places, but that strikes me more as an interface thing than a backend thing. I mean, lets look at IP.B's backend: loading a simple thread takes about 11 database queries*. It's not exactly the fastest board system around either, and it handles a lot of template issues in seemingly retarded ways**.
Overall, I hate both boards. Sometimes, I open a piece of software and I just get this feeling about it, like it's going to break apart in my hands if I push it too hard. So far, both boards have given me that feeling, and both continue to do so (although I have yet to try their newest versions). SMF (what I run my own tens-of-thousand-posts board on) gives me a similar feeling. Actually, I could sum up by saying that I hate all board software I've come across yet (except for the now-abandoned
IceBB; I didn't like it that much, but it looked like it had promise). Given all that, though, I think I'll conclude by saying that of both options given, I hate IP.B slightly less than vB.
--- Mr. DOS
* IP.B 2 numbers here.
** Again, IP.B 2. I hear tell they've made a lot of template system upgradifications in IP.B 3. I sincerely hope so - it couldn't have been much worse in 2.