Unless there is some specific feature of the OS I want to beta test, I always wait until release.
The one feature I want was talked about on RedHat's feature tracking system -- BitLocker-like functionality if the machine has a TPM. What does this mean in nontech terms? It means I can have a server boot unattended without needing a password to start up, but if someone pulls the hard disks out and tries to access the data, its all encrypted. For most people, this doesn't sound like a big thing, but for me where I have had to locate servers in very remote (and relatively physically insecure) locations, it is an excellent safeguard.