Also pain split is VERY unreliable and shouldn't even be counted as a means of recovery. This is coming from a long term Dusknoir user :P
Gotta disagree. I've used dusknoir alot, and I find pain split to be overpowered. The enmy just can't't get rid of him. It requires a bit of prediction though.
For example, the enemy sends in Kingdra. It's a bad match-up for you, so you switch into dusknoir. Meanwhile, dragonite dragon dances.
Kingdra attacks you, dealing 56% of your life. You will-o-wisp in response. Your leftovers heal you, you now have 50% left. Dragonite loses 12%, and has 88% left.
Kingdra stays in, to keep it's dragon dance, and attacks again. You lose 28% and have 22% left. You predicted that and pain split, and both have 50% left. Kingdra loses health from burn, and you gain leftovers recovery. After this, it's not hard finishing him off, although you will have little health left when you're done.
You have 28% life left. Enemy sends in someone who tries to revenge-kill dusknoir, but dusknoir's bulk can take it (28% is actually usually less than what he has left after finishing off a physical attacker). He then pain splits, and is back at 50%, while the enemy also is damaged. Then you can either will-o-wisp or switch out.
Thing is, pain split is almost impossible to switch into, unless you have a damaged pokemon. Even if kingdra had switched, you would've gain lots of health. And as soon as you've killed a pokemon with dusknoir, the opponent will most likely only have full health pokemon left. Even if they see a pain split coming, they won't be able to do anything about it.