Related to the Outnumbered sitcom which is semi-improvised. I personally see writing the dialogue as part of the writing craft and the Office and Royal Family as a perfect example of writing real dialogue and Alan Partridge to some degree too.
I can see why Spinal Tap did it so well as it was supposed to be a documentary, but surely in a sitcom it is better to speak weeks thinking up of funny lines rather than letting someone come up with them on the spot.
I think the BBC have been quite clever in the fact that they really have pointed out that its semi-improv: the newsletter, tv listings and before the show started all mention it. I think it lowers the expectation a little so that when a joke is made you think its better than it maybe.
Anyway, just my thoughts, I'm open to others.