RE: yossarianuk
A lot of people want stability. Stability that lasts for years. RHEL (and CentOS) give you seven years of platform stability for each major release. Sometimes that means you will have "old" kernels or not-quite-bleeding-edge PHP by default but for a lot of people that doesn't matter - as long as they get bug-fixes they *like* knowing that the versions don't change for years on end.
If you have a project that depends on particular library/API/versions that might last five years or more it's an excellent choice. If you want more rapid turnover then there's Ubuntu or the like.
A lot of commercial (closed-source) packages are also built against RHEL versions (sometimes with SLES) and if you don't use them you're on your own. The application died after running a three-month long simulation and you lost everything? Too bad - at least you've got the latest compiz & gcc to play with...
I'm not anti-debian or anything - I happily run RHEL&clones alongside SuSE, debian, Ubuntu (and Solaris, AIX, IRIX..). Different OS's for different cases.