In pve I use ambush as an opener, when I crit with snipe/SoS or if my energy is close to 60%, as this will give me time to regen a bit while still putting out nice damage. pvp it's only after snipe or SoS crits only. Good use is if you get stunned by a melee, stun them back, instacrit snipe and follow with an ambush, just enough time to send them flying (if you've specced it), giving you time for 1-2 more snipes/other good stuff.