Does taking One fo Many have any impact on the story? If it does, what is it? Is this NPC worth taking (I'm a combat orientated character)? I guess everyone needs a rogue sometimes, but it seems NPCs here don't gain XP even if not with you, and since I wouldn't be taking him (it?) too often, and he as he can't use/carry items maybe he's not worth of attention...
I believe he is involved somehow with how you handle Myrkul. I wouldnt bother with him either. There is only 1 door/container in the whole game that's locked and requires a pick lock skill higher than Kajji (sp?) has.
One-of-Many can have a big impact on how the game plays out.
First off, you can change it to 3 different jobs. He starts out as a Rogue, and once you get some influence with him (don't remember the exact value) you can change him to a Barbarian. If you allow him to devour the Sleeper in the Skein he will be able to become a Warlock.
You can allow One-of-Many to devour Myrkul, which will lead to Myrkul taking control of One-of-Many. He will tell you that he wants to meet you alone, and if you do he will attack you. After defeating him you can then choose to continue travelling with him or devour him.
I'm not entirerely sure what's needed for this, but in some ending (I think maybe if you romance Safiya?), One-of-Many will kill Safiya.
As for if he's worth taking, if you're playing evil I would say absolutely. And I would suggest trying that, as MotB has one of the best evil playthroughs I've seen, and I normally find them very lacking.
I don't really know what check are you talking about since I never let One-of-Many eat him, but you can test if any diplomacy check is possible by cheat-adding yourself an absurdly high charisma. SetCHA 90 would probably do it.
I haven't checked myself, but I would guess that the diplomatic check is impossible. I didn't succeed, and I was playing a character which I had pretty much dedicated to maxing out the diplomatic skills as much as possible.