We use Debian as one of 3 standard platforms (the others being Sun Solaris and HP Tru64) for members of the department and it is the only standard platform for our student labs.
Currently we maintain more than 200 Debian workstations (user desktop machines) and more than 20 Debian servers (samba, database, web, print ...) in a semi-automatic way: all are kept up to date based on a few master machines. The masters are installed and configured manually, but distribute their state (installed packages and configuration) every night automatically to the others.
We have been doing this with Debian since 1997. The reason for choosing Debian over other Linux distributions are obvious : ease of package administration, smooth upgrades, installs, ... other distributions might offer other software packages, but none offer the same degree of quality ... if it's available in Debian, it just works.