We have migrated our 8 servers from various commercial Linux distributions and OpenBSD versions to Debian. These systems do fileserver tasks (Samba), groupware (Lotus Domino/Notes), development backend (Subversion), Apache2, LDAP, SquirrelMail, database backend (DB2, Oracle, PostgreSQL, MySQL), virtualization (hostsystem for currently 12 virtual machines running inside VMware Workstation), firewalling and traffic accounting, monitoring (Nagios, MRTG, smartmontools, AIDE, Snort), backup services (rsync) and so on.
I decided to use Debian because I have some years experience in using it. Package management is superior, administration is really easy because of the excellent structure inside /etc and updates always work like a charm, even dist-upgrades do, and all systems run stable as rock even with average system loads around 7.0 24/7/365 on some machines. Incomparable to other distributions we had running before, now there is more time for the nice things in life. We plan to migrate several workstations from Windows XP Pro to Debian Sarge as well, main applications will be Java development (Eclipse and Websphere, several database servers). But there is no timeline yet, so my workstation is currently the only one running Sid.