Moderatly useful. There isn't really an objective scale to measure it against, so really that's the best you can get.
Look at it like this: it's a situationally useful ability, the utility of which scales with the number of people it's operating on. In a standard adventuring party of 4-6 people, it's rather hum-drum, but with the nice feature that Down no longer necessarily means Out. You can drop to negative hit points and since you're still able to function, you can pop a healing potion, withdraw from the fight, take cover, cast a spell, or whatever. Not bad, particularly because it's one more thing standing between a resource-valuable PC and death, but not blow-your-socks-off amazing.
On the other hand, if it's being used by an officer in command of a hundred-man strong force, all of a sudden the utility increases sharply. Now each one of those one hundred soldiers has 10 extra hit points. In a large scale battle, that's a big deal. Means things like if an given combatant can take two hits and stagger away, or three hits and go unconscious (generally tantamount to death), your guys can all of a sudden each take four hits and still have enough in them to fall back and get stabilized, and it takes five hits to actually kill them. All of a sudden, your soldiers are each worth two of the enemy. On a grand scale, abilities like that win wars.