A few months back, I took on co-system administration duties for one of our groups most critical data and application servers. It provides several important services, including housing our back-end test harness database, maintaining the Bugzilla database for Sun's Secure Application Switches, and acting as the main execution server for our test automation. Until recently, it was cranking away on a five year old system running an ancient version of Red Hat (7.1 I believe). In desperate need of an upgrade, we got a hold of an X4200 to replace it.
The X4200 class of servers is certified to run Solaris, Red Hat Enterprise Linux, SUSE Linux Enterprise Server, and various editions of Microsoft Windows Server. Flying in the face of logic, we chose to install Debian stable (along with the custom kernel compiling that was involved to provide support for the X4200's SAS disks). There are a lot of advantages to running a Debian system, most notable its superior apt-get package management system. After some initial aggravation setting it up, it has performed with minimal complaints since.
Our situation is somewhat unique, however. We have just one system to support with only a small set of functions. Most corporations, though, would not choose to buy racks of expensive Sun hardware and then run an unsupported operating system on them. Those who want to take advantage of the benefits of a Debian-based distribution are on their own. It appears, though, that this may soon change. According to reports, Sun and Canonical are ready to announce plans to certify Ubuntu (a Debian derivative) on Sun's x86 servers (including X4200's).
Yet another step in the right direction.