I recently had the opportunity to play around with x64_dbg as OllyDbg can only debug 32 bit executables. In fact this is not exactly correct as there is a version 2 of OllyDbg that supports x64 but it's still a work in progress (last updated in February 2014) and I didn't have much success with the apps I tested.
My initial impression is that any OllyDbg user will immediately feel familiar with x64_dbg and quickly get debugging. Of course x64_dbg being a relatively new project it only has a few plugins so far. Anyway try it out if you have any 64 bit apps laying around.