Viper Sting will break Freeze Trap, too. I put that to a test one time, thinking it'd be a great way to assure some mana drain.
Personally, with the cooldown added, I haven't been using Viper Sting nearly as much. It used to be that you could fire another one immediately if the target cured it, but not anymore.
Before that, I found it most useful against paladins. For stings, though, I typically use Scorpid if they're melee, or Serpent for casters.