I use vBulletin 4 and I absolutely love it. I've also used phpBB and IPB.
The more I've learned about vBulletin, the more I appreciate it - so customizable and dynamic. I never thought I'd be able to write my own mods for a forum to make it exactly how I imagined.
So my list:
- Easy to skin & make it look very professional / attractive
- Can integrate Facebook-esque mods that keep users engaged (Liking/Instant Notifications/Etc)
- The CMS works great (with some modifications)
- The whole plugin / template system is amazing. Updates don't break a thing.
- vBulletin.org is extremely helpful
- Our users seriously love the forum software. We get constant comments about how easy and intuitive it is
- Extremely secure / safe from spammers. Never been hacked. And after installing Spam-O-Matic, we have never had one spam registration.
- It's a really fun playground

I'd be bored without it