We are using Debian on 100% of branch office servers (75 units), 100% VPN routers for external ATM (15 units), 80% of central servers (43 units, distributed systems, non host part), and 40% of desktops (160 units).
These data are dated end of 2010. Since then, Caja de Guadalajara has joined Cajasol, and Cajasol has joined Banca Cívica in 2011. Technological resources are currently being reorganized, so, we can not write exactly amounts of systems with Debian.
We have chosen Debian because of stability, functionality, level of development, flexibility and adaptability. We like Debian.