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I use it in PvP all the time, it doesn't really require a lot of stamina to use, and in small scale PvP its a game changer. Nothing better than reflecting crystal shard back on someone.
I personally wouldn't use it in PvP. Stuns aren't that useful against a skilled player and it would need to be reactivated regularly to be useful against casters. As it only works against magical projectiles it is useless against AoE attacks which are fairly common in PvP.
The heal morph is probably the more useful morph for PvE as a sword and shield user is likely going to be a tank and not concerned with DPS as much as survivability. But again if a boss is launching an AoE attack it won't have any effect. The block cost reduction could prove quite useful however.
Just my two cents based entirely off of my knowledge of the game. I have not used this ability in either scenario.