He's the best of several other characters without their downsides and still having his own original twist that's fun as well.
He has:
The idea of crafting from tainted Cain without the unbearable annoyance of not getting items naturally (yes, I despise tainted cain).
A high damage and fun unique attack gimmick like the Forgotten.
The idea of consuming items from tainted Appollyon but what you get is actually creative and worth it instead of a generic thing no one cares about. It doesn't even have to be unique for every item, even just categories like "all mom items do the foot thing" just does a lot for it. I also love the little cosmetic combinations you can get for the bags.

Tarnished Cain made me hate that we got tainted Cain instead when this is this good in comparison.

The only thing I don't like that much is how the bags picks up bombs, I found it more annoying than useful as a lot of the time you don't notice it and it explodes on your face.