[Hellman2]

Here the level descriptions:

Level 1: Revives 2 allies + Restores 25% of their health
Level 2: Revives 3 allies + Restores 25% of their health
Level 3: Revives 3 allies + Restores 50% of their health
Level 4: Revives 4 allies + Restores 50% of their health
Level 5: Revives 4 allies + Restores 75% of their health

So before level 3 not worth it, because its just giving free RP to the attackers.
The range is not really defined where it works. But if you do it inside of forts (like on tower) you may revive dead ones at the gate (at I saw it once working). Who is revived may be random.. not sure if it will be the nearest ones.