We are a commercial law firm and have been using Debian since 2002. It basically started off as a backup system. However due to its stability, resilience, lack of virus sensitivity it has taken over as main server and now handles mail serving, DNS, acts as a file server, database server and fax server. We use 4 TB disks in a dual RAID 1 configuration.
Little maintenance and ease of hardware substitution and migration along with ease of backup has allowed us to use Debian exclusively and has proven itself to be extremely good especially as a mail server (Postfix, Dovecot, Roundcube). It offers unparalleled ease of updates and upgrade via the apt system.
Debian has proven itself to be an ultra reliable partner in a business environment where data integrity and lack of virus related downtime becomes mission critical.