CentOS describes itself as the community enterprise operating system. CentOS 5 was released in April 2007.
Cranfield University has been using CentOS in lieu of RedHat Enterprise Linux for about two years now. We have it deployed on approximately 100+ dual-boot Linux/XP desktop PCs plus some 48 compute servers in a compute 'farm'. CentOS is directly equivalent to Scientific Linux (SL) but comes from a community-base rather than the two scientific institutions who support the SL distribution.
We have found it to be utterly reliable and the strength of the community support model is evident on the few occasions we have had to invoke it. We do, however, restrict its use to engineering computational activities rather than business-critical ones. For these we use supported RedHat Enterprise though we are confident that CENTOS would do an equally good & reliable job.