Holy Shock is sadly limited in my experience, and its efficiency is really quite poor. The use I can find for it is casting it right on the end of a normal heal for a kind of manual critheal, if the tank is getting bursted down that little extra couple of k can really help out. As a healing mechanic though it sucks, it has a wide and varied stack of disadvantages like short range (20yds compared to 40yds for every other thing you cast), high cost, long cooldown and a poor +healing coefficient.
The other thing is, for PvE anyway, that since it is instant it uses up a global cooldown, so casting it effectively costs you 1.5secs of command time. That's the same that Flash of Light costs you, which is a far superior choice for healing.
Like I said, nice for emergencies after a primary heal, and I guess Shockadins have fun with it. Other than that, meh.