We run Debian for everything, from desktops to servers. We have application servers over tomcat (running ERP), web site, e-mail, firewall, proxy, file sharing, telephony/VoIP server, and a NMS. Everything running on Debian. The total is 16 virtual servers, split in two physical computers for high-avaibility.
We choose Debian to ease the burden of system administration. Also because Debian has very well maintaned packages and for high security (and of course stability, that we include under security for logical reasons).