CentOS 2 and 3 are a 100% compatible rebuild of the RHEL 2 and 3 versions, in full compliance with RedHat's redistribution requirements. It is for people who need an enterprise class OS without the cost of certification and support.
I've just installed it on a spare box and it *appears* to be everything it says it is. It'll even install under VMware (I have Workstation 4.52) so you can try it out even if you don't have a spare machine lying around.
Cheers
ZB