Scientific Linux is really good, but it has issues being updated from one version to the next using the "Yum" command. CentOS has been really good in 2012, but has been highly variable over the years.
Another RHEL clone I haven't paid much attention to is Oracle Linux. While it was free, you had to pay to get security updates. Well that has recently changed and, indeed, it's possible to switch from either CentOS or Scientific Linux to Oracle Linux without reinstalling. This is especially useful for cheap OpenVZ accounts, since one can install an ancient template of CentOS or Scientific Linux, update it to Oracle, and have an up-to-date RHEL clone.
In terms of people looking for good work, I once had a phone screening from a clueless HR drone that went like this:
"Do you have experience with
"Yes, I have used RedHat since RH4.2 in 1997 and use both CentOS 5 and CentOS 6 on my servers."
"But CentOS is not Red Hat!"
"It's 100% compatible"
"Sorry, we only want people with recent Red Hat experience"
Now that Oracle Linux is free, it's a lot easier to get past a clueless HR drone with the buzzword "Oracle" than with "CentOS".
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