I have always left the default vBulletin emails that my forum sends out. (i.e. to activate a new account, inform a user of a new private message, etc.) However, I finally took the time to customize each e-mail - and I can't believe how much it's helped!
The vBulletin forum system is so common nowadays, I guess I never realized how many people get the exact same sterile e-mail from so many forums. Customizing the e-mail makes the forum more welcoming and has really increased the number of new registrations that actually post.
Just a helpful hint ;)