I don't know how many people have noticed that the second rank of this enhancement is one of the very, very, few things in DDO which gives completely unlimited healing. Well, repairing.
The tooltip seems to be very wrong. For Rank 2, it claims the iron defender repairs itself every 12 seconds (true) and that it also repairs you every 6 seconds (false). It repairs both every 12. Rank 3, it seems to repair itself every 6 seconds, and me every 12. And that does require construct essence or being warforged. BUT.
So this isn't gonna exactly win any speed-healing competition prizes. But... It's still unlimited healing. No spell points to run out. If you can get somewhere quiet, and wait, you have full health.
The only other completely unlimited healing I can think of is the eternal wand of cure minor wounds. I think the dog's better.
Cost: 13AP and at least a couple levels of artificer.