the BBC version or the american version.
IMO, enjoyment of the BBC version hinges on what you think about Ricky Gervais, because it's basically just him improvising his way through uncomfortable situations and making them worse. It is what it is. THe other characters help anchor it a bit, but it's basically the Ricky Gervais show.
The american version isnt quite as anchored to the lead (although in certain episodes it's basically the same deal--if you like what Steve Carrel is doing, then you'll enjoy it. If you are annoyed, you'll hate it). The main difference in the American version is that the rest of the cast and characters are much more important to the show.
I think it gets a little too outlandish and cartoonish at times for my taste (i had been hoping for something that was a more realistic play on office life, not on eccentric characters who happen to occupy an office). But if you jsut go with it even some of that can be pretty inspired.
I dont pay much attention to the "documentary" thing going on. i think it's an albatross on the show, but they seem to get around it pretty well. If you focus on that too much then the show ceases to make sense because often the cameras are in places they never could be, and characters are showing their true stuff much more than they would be if there was a documentary crew there.