We began using Debian in 2003, for implementing network servers for our customers ranging from small to medium size. All are based on the Intel x86 architecture, bringing them file services with Samba, DHCP server, firewall and proxy server, SSH server for remote administration, etc. We also use Debian on our server to handle centralized services like transparent proxy with Squid, IPTABLES, SSH, DHCP server, intranet web server (for testing purposes).
I started with SuSE 6.1 in 2000, because it had good support for Spanish in Venezuela; that was my first Linux experience. After that I met Debian in 2001. I installed Debian, and started to learn and use it, and the more I learned the more I liked it. So, we started to use Debian for production in 2003.
I chose to use Debian because it's reliable, it has an excellent software management with the apt, it's really free and open. Since then we use open source software whenever it is possible.