I see people complaining about some kind of mouse look issue in Synergy so here is the version I compiled. I have been using it for a couple years now so it does not have whatever latest and greatest enhancements exist in the current Synergy base, but it works well for WoW.

http://avatar.telefragged.com/synergy4wow.zip

When you hold a mouse button, it keeps the cursor in the same place, so you can mouse look around fine.