This movie moves at such an incredible speed it almost works. The performances are all great (particularly, surprisingly, Christopher Reeve, as the cleverest man in the film, although he wishes he wasn't,) the pratfalls are all funny, many of the lines are supberb, it is all incredibly timed. It is everything a good sex farce should be. Except...
Except it doesn't build. It goes in the reverse order to what it should - it puts all the funniest stuff in the first act, has a second act that is funny when it makes sense or someone is falling over (and very funny when both are happening,) but the third act just kind of sits there. It doesn't build. I mean, the situations get progressively more ridiculous, but they don't get any funnier. They just get to a point where it is all so ridiculous that all the characters have given up trying. And when characters stop trying, farce stops working. You just sort of sit there, thinking, "this would all be a lot funnier if they were just performing Nothing On properly, without stuffing up. Most of it was funnier in its original context."
Maybe that is the problem with the film - it seems like the play that the troupe is staging would be funnier than what we are watching. I just want to see the second act of that, rather than just progressively worse versions of the first act. I mean, yes, it's a very daring and meta thing for a play (now a movie) to attempt, but you're not Tom Stoppard, Michael Frayn, you're writting plays about sex and people falling down a lot. Save the deconstruction to people who know what they're doing.
Maybe I'm being to hard on it. After all, the only performance of it live that I've ever seen was a High School Play that one of my friends was in, and it was funny, but, you know, a High School Play. Maybe the building ridiculousness of the final act just simply works better on the stage than it does on film. All of the reviews from the time seem to think so... I apologise, Michael Frayn. Also, I just checked your Wikipedia page, and found out that you are now a successful dramatist, as well as novelist, and translator of Chekhov and Tolstoy. So, yeah, that Tom Stoppard crack was maybe uncalled for. Sorry.