WTC Communications, Canada
We have been using GNU/Linux for over 10 years. We have, over time with upgrades and expansion, replaced all of our systems that are not dependant on another distribution or operating system with Debian. Our main reasons for jumping over to the Debian ship are decent out-of-the-box security and how straightforward and standardized management of servers became: the releases are very stable, the package selection is huge, there is plenty of community documentation and thriving peer support groups, support across multiple architectures is good (allowing us to have a standardized installation on both x86 and UltraSPARC hardware), and of course the incredible APT package manager. An added bonus is the no-nonsense 'netinstall' image which greatly reduces setup and configuration efforts.
Currently our install base is over 20 production systems and growing, with a variable number of development systems popping in and out of existence. Being an ISP and telecom we use Debian in a variety of tasks, including Apache web servers, PowerDNS and BIND DNS, mail systems on Sendmail and Cyrus, virtual machine hosts and guests with VMware, MySQL and PostGRES databases, OpenVPN and PPTP VPN systems, software routers and walled gardens, dedicated firewalls, performance monitoring with SNMP logging and Cacti, file serving with Samba and FTP services, attaching to Fibre Channel and iSCSI SAN storage, backup systems, subscriber authentication with FreeRADIUS, network management such as RANCID and TACACS+, CRM, ticket systems... the whole nine yards!
It seems that with other distributions, such a wide array of functions would be difficult to tie back to one standard, flexible, familiar base.