The vulnerability is quite high, but it is exploited when you serve time, not when you merely sync from another server. The issue with ntpd is that the same program acts as both a server and a client, so having it running at all means you are a potential time server (other clients can connect to your "server" and sync with your time, or do bad things).
OS X's default config file has had this locked down for quite a few versions now (Snow Leopard is the oldest I have on hand to check). This lockdown (the "noquery" config statement), mitigates 5 of the 6 new vulnerabilities without any patching needed. And the sixth involves configuration changes to the default OS X config that most folks are not even likely to understand, much less invoke.
Beyond that, if you are behind a NAT router, your ntp server port is not open to the Internet to be exploited, unless you specifically port-forwarded to it. Though it could still be exploited on a local LAN.