"Brother!" The shout shout cut through the night, the shrieks of a thousand ravens. Beneath the trees, a man muffled head to to heels in mottled blacks and greys sat astride an elk. "Here," the rider called. A hood shadowed his face.
"He wasn't a green man. He wore blacks like a brother of the Watch, but he was pale as a wight, with hands so cold that at first I was afraid. The wights have blue eyes though and they don't have tongues, or they've forgotten how to use them."
I think that's the extent of what we know about Coldhands. I think it's suggestive that he calls out 'brother' to Sam to grab his attention. It doesn't necessarily mean that he considers himself part of the the Watch, but we can't rule out the option either (and Sam does say that he wears blacks like a brother would).
So TK, to take your objections in order:
Well, for starters his description does not fit Benjen and Bran would of recognized his missing uncle.
First and most importantly, Bran never meets Coldhands. We cut away from his POV as he's passing through the black gate, on his way to meet him, Bran never has the chance to recognize him. Secondly, the description of our mysterious stranger is so vague that I don't see how we an disqualify anyone... We have no indication of his size, his face is hooded and shadowed, all we really know is frozen black hands and he doesn't have the piercing blue eyes of wight. That could be anyone.
Coldhands cannot cross the Wall or go under it through the gate at the Nightfort, which suggests some magic involved. That would also exclude him being Benjen as a member of the Nightswatch could do so.
I guess my thought is that some magic (Children of the Forest?) prevented Benjen from dying and becoming a wight and turned him into... something else. He can't cross the gate because of the same magic that keeps the Others from passing.... but he's still fundamentally Benjen Stark....
The general feeling is that he is working for the Children of the Forest. Or that he is working for the Three-eyed crow. Some have suggested he is even working for the Others that this is highly unlikely. Some say he is wight of some sorts.
That all seems perfectly reasonable to me. As I've said, my first impression reading through was that he's a wight of some sort, working with the Children, and Benjen.
Many have just said that Coldhands being Benjen is too predictable and GRRM would not go there.
I don't really put much stock in anything being 'too obvious' or not. The very fact that we come here and post on this series means that we're paying way more attention and giving way more thought to the books then the vast majority of the people who will ever read them. At this point, every reread I do R+L=J seems blindingly painfully obvious. But I didn't notice at all my first time through the series. And there's a bunch of stuff like that. Obvious is neither here nor there.