I've moved over from CentOS 5.6 to Debian 6 as I felt like a change and wanted to learn more, so from now on I'll be concentrating on Debian and documenting my progress with it, or not, as the case may be :)
A lot is similar but theres slight differences, like you can't log into the root account from the user splash screen where you log in, but you do get more up-to-date programs like kmymoney is version 4 in Debian whilst centos only has version 1. Gramps is more up to date too.
Overall I like Debian 6, as it appears to be rock solid and stable, like Centos, but more so. And its more desktop-orientated, which Centos fell down on.