I have it running on RH 7.0.
I'm using ntp.ourconcord.net and nss.nts.umn.edu
I initially had problems because my firewall was blocking the UDP packets that the time server was sending.
Check your messages log for denied packets from those servers.
Also your machines local time must be within 1000 seconds or it just won't sync.
It takes a while to syncronize, after about 15 minutes, you should see the time on the machine has shifted and locked on to the correct time.
You can use the ntpq program to debug problems. See /usr/local/docs/ntp-4.0.99j/debug.htm for instructions
Also if you reboot your machine it won't automatically re launch unless your tell it to start that service at startup.
In RH as root run setup then go to system services and set ntpd to on.
hope this helps.