[1] With the exception of Germany, where SUSE Enterprise appears to be most popular.
I guess only the big old customers stick with SuSE. I don't know a single person anymore using SuSE…
I've recently helped to install a new server farm for a media company. We've choosen Ubuntu 10.04 LTS + chef (opscode) + REE + kvm.
We found that stability to be on par with CentOS, with the primary advantage of Ubuntu being that we have more up to date packages in the main distribution.
We have a number of people running Ubuntu on their workstations which was another benefit of "unifying" the underlying platforms.
BTW, yes apt is awesome
(I assume we're not talking desktops here.)
I find that debians support cycle for stable is troublesome for long lived servers, as the support lifetime is unknown, other than it's a year past the release date of the next stable version. That means that the minimum lifetime before needing to recheck everything for stability is one year plus however long until they release a new stable.
In contrast, ubuntu is at 5yr support for their longterm releases, with a known 2 year cycle in-between. Which means that even if you install in the worst possible time in the cycle, a new install will be supported for three years.
I really like those extra two years of security updates.
(edit for iPad text entry)
Debian is great for one or two machines, but based on my experience with it I can't imagine using it for 10,000 boxes
As for requiring a very specific configuration, most of the time that is only in one corner of the machine. I would even hazard that if you have strict requirements for every facet of the machine that differ from the defaults (and this machine is not handling super-sensitive information, super-high performance jobs, or publicly accessible) then you are doing it wrong.
In any event, another good reason for running RHEL is when there' a legal requirement that you can't use "freeware". Mostly the place I'm at develops on CentOS and then pays Red Hat for what we ship simply because then we'll have paid someone for what we ship.
o CentOS - which is pretty bog standard for large web environments that are RPM Based that need to, for whatever reason, stay close to the RHEL platform.
o RHEL - For anybody who needs RedHat Support, or needs to be able to tell Oracle (or other enterprise vendor) they are running on a supported platform. Or they are just PHBs that need the comfort of deploying "Supported" environments.
o Ubuntu - This is a bit of an outlier, but we have a number of Debian Snobs who have a preference for the apt/dpkg/deb way of doing things, and Ubuntu, particularly LTS, seems to have some mindshare here.
Nobody mentioned Suse, though this is in Silicon Valley, and I get the sense that Novell doesn't have as much presence here.