Our college uses Debian in the six HP Blade30p, two ML350, and in one TB NAS box. We are running Kerberos, OpenLDAP, Samba, Exim, Cyrus, SquirrelMail, Mailman, PostgreSQL, Bind, Apache, Squid, VsFTPd, sshd in these servers. Apart from these we use Debian in three other lower-end HP servers [NIS, NFS]. The number of client workstations that houses Debian would be around 110 in the Central Computing Center and another 35 computers in the Parallel Processing Lab of the Computer Science Department.
The reason for using Debian is due to its stable packages. It is also easy to maintain and upgrade.