We run a system of about 40 machines based on Debian. We have a set of servers (NFS, NIS, DNS, web server, mail server, FTP) running Debian. The system is used by about 80 users for mathematical research as well as Internet stuff (web browsing, email, FTP, etc.).
The most useful things I found about Debian are the big number of packages, security (excellent, fast upgrades) and the painless upgrading method.