In my experience I've seen it used in servers quite a bit, typically those running cpanel. Its very similar to RHEL but without the pay-support plans. I've helped to run a machine running it for 3-4 years now, with no real issues.
If you need any software that depends on relatively new libraries - forget about it. Centos is seriously behind with features. It could be useful for "standard server" though - stuff like http daemons, etc. will run just fine and will be quite stable.
Not for home usage though - I don't think you'd like to run on something with kernel v2.6.18-92.1.1.el5 ;) Yup - that's kernel from 2006 with ~100 custom patches applied. Forget about any new gfx drivers or wifi.
I use it in combination with perceus to run my cluster. very smooth install, but I don't update it very often. Best thing for that sort of application, in my opinion.
