Debian is used in our organization as our primary flavor of Unix, and is installed on all of our servers (file, print, backup, web, ftp, email, Samba, netatalk) and the Unix workstations that scientists use. In addition, we have a (currently) 12 node cluster running Debian. In total, Debian is running on almost 40 desktop and server systems, as well as more than a dozen laptops.
I chose Debian because it has a superior package format that keeps track of dependencies and is easy to use remotely, and because of the number of packages available (more than 10,000 right now). The bug reporting (and fixing!) system is excellent, and the volunteer help I've received over the years has been incredible.