Some people like CentOS, some people like Ubuntu -- it really depends on what is important to you. If you don't mind having to source your software from 3rd parties (DAG, RPMFORGE) or compiling from source then CentOS is a good choice. Ubuntu definitely has the edge on the number of precompiled packages. I use CentOS mostly out of the fact that I started with Redhat 3 (not enterprise linux 3, redhat 3) and a good deal of my knowledge is sourced in Redhat/CentOS.

Also a lot of the CentOS packages are outdated (it's not CentOS's fault, it's Redhat's) but still I think the version of Postfix which is included with CentOS 5 [2.3.3] is several years old.