I have two WordPress Mu sites, both for teachers at the High Schools in Logan County, KY. One, my wife’s school, I had delimited on registration domains to allow only those with appropriate email addresses to use.

The other I had not.

Yesterday I received a very polite email from a gentleman asking me to remove his content from one of my blogs… and I found to my shock and horror that I had about 150 spam blogs and over 300 spam users on that domain. It was using more than 400% more of my bandwidth and disk storage than any other site on my server. I just had not been paying attention.

Moral of the story: No Mu is good Mu?

EDIT: I just want to point out that I am not going to stop using WordPress Mu.  Bofe is right (as usual), I just needed to have  it set up correctly.  I was just sharing a very surprising experience.