it's kind of a mixed basket:
the healing portion of it is virtually neglible for Holy, it's not really a dps strategy, but it is a great excuse to be in melee to get mana back from Seal of Insight which you may already be doing, and it jives well with Ascetic crusader to the same purpose (summary : Mana tool). it would be a different story if it transferred something like 100% of the damage from melee and CS and Judgement for Holy paladins.
Ret gets the most healing done out of it but it will be hard to convince rets to forgo dps of their seals of truth and righteousness to use seal of insight.
For Prots it works pretty well, since some Prots run Seal of Insight for the mana and self healing when threat is not an issue, (and it almost never is an issue for them), this just adds a little bonus to the group, and it's a smart heal so it quite efficient.