Yes, we add the following repositories to all RHEL5 installs:

EPEL
FC6 Extras
freshrpms

These seem to be the main "safe" general purpose repo's.

I have considered CentOS several times in the past as RHEL licenses cost quite a bit of $ each year and cost cutting has been the "in" thing business wise ever since the dotcom bust.

Still, we stick with paying RedHat for RHEL licenses each year, because:

a) Most of our deployments are on VMware ESX whose release notes on the Linux side are constantly focused on RHEL. Debugging issues can get very complicated in a private cloud, and we just can't afford the added confusion that might occur if we contacted VMware/Dell/Random Application Vendor and told them we were using CentOS rather than RedHat.

b) I've been disappointed many times in seeing very long wait times between when Redhat releases a patch/upgrade and when CentOS finally gets around to repackaging it. When a critical security patch is released, even an hour or two can be important for deployment.

c) While I honestly would prefer that my primary distribution be community based, I would expect the community to be innovating/etc. I'm not sure how I feel about centos basically just being a clone of redhat. Free is nice, but I feel better compensating RedHat for the value it provides.

d) Telephone/Email support from RedHat isn't really a factor. In the last 10 years, I might have had to open a ticket perhaps 3 times. Most other issues were handled without even going to the redhat website, although sometimes it is helpful to read or update bugzilla.