I bought a garmin refurb 665 from an authorized seller off ebay and sold the XM unit so the 665 only cost me $500 essentially brand new. It spent a week crashing and freezing at every opportunity and is now on it's way off to Garmin. MP3 would crash it, the phone would crash it, routes would crash it, I swear passing birds would crash it.
My advice is where ever you get one and from whomever you get it, make damn sure it works fine or you can return it or get it sorted out under warranty. I never had any trouble with my old 550.