I contacted a pal who was lots of this kind of experience. Well, IT depts. The easy thing with a log is that you can basically run a Cyberpatrol or NetNanny style database through it. That means that you're doing automatic pattern matching on tens of thousands of porn domains to find the culprits. So you don't have to sit there eyescanning 100's of megs of files when you should be fixing printers, the computer will do it for you automatically, and with a firewall it can do it in real time.
Big offices block porn as well as many less sultry entertainment sites.
All email can be logged and scanned with a computer as well. You have to really be careful.