I first watched the UK version and loved it, then watched the US version, and to be honest, the UK version has never felt the same anymore Usually the UK ones beat the US ones, but this time it's the other way around, at least for me.
I tried watching the US Office the other night, just to give it a go. I'm afraid I found the acting far more forced and unbelievable. With the UK Office, the first time I saw it I actually thought it was a real documentary for a while. I would never have thought that with the US Office.
I know what you mean about the UK Office first seeming like a real documentary, I too thought it was for real and thought 'good god, the boss can't be serious!!", and the US version doesn't have that quality. But in so many other ways it's superior!