I never actually knew much or looked into forum softwares before but this thread made me to. Google came up with some reviews and they give an overall impression. Sorry to say most user comments talk about same things deathshadow mentioned.

vBulletin3.8 user rating 5/10 of 83 votes.
vBulletin4 user rating 3/10 of 37 reviewers.

You can look up at:
http://www.forum-software.org/vbulletin/user-opinion
http://www.iterating.com/component/c...7501-vbulletin

This just for quick Googling I'm not saying you should change or something because of this, just stating the facts. It may be big, slow, bloated resource hog but it's used by alot of people.