We use Debian since 2004-2005. We have 30 servers currently running Debian Lenny. The services we provide are: Academic system (PostgreSQL database, Apache2, PHP5), DNS (Bind9), monitoring (Munin, Nagios3, MRTG), authentication (OpenLDAP), file sharing (OpenLDAP, Samba), virtual machine (Xen), mirror and other services (rsync, Apache2, Git), webserver (Apache2, PHP5, PostgreSQL, MySQL, Drupal 6), streaming (Icecast), security (Fail2ban, iptables).
We chose Debian GNU/Linux for these reasons: Stability, easy security updates, easily downloadable packages with a mirror leaf in our network, customizable to our needs, and the variety of packages in the distribution. Thanks for the great work you are doing.