I found one !

Yes, after two tiresome hours of net hunting, I found a newspaper review in which the author seemed to have actually enjoyed Mr & Mrs Fitch:

In "John Lithgow, Jennifer Ehle star in wickedly funny "Mr & Mrs Fitch" off broadway", the author Robert Feldberg tells us that

Played with great panache by John Lithgow (which you might expect) and Jennifer Ehle (a fine dramatic actress unveiling a shining talent for comedy) [...] what makes the play such fun is the seamless collaboration among writer, actors and director Scott Ellis, who never let's the evening momentum flag.

The Fitches' great shared joy is their love of language. They never use one word when 10 will do; they don't speak plainly when it's so much more satisfying to be clever.

Second good news is that we have a video of Mr & Mrs Fitch premiere party which took place on Monday, February 22, in which Jennifer Ehle gets to speak about her role, and some nice pictures of it.

(Director Scott Ellis, Actors Jennifer Ehle and John Lithgow, playwright Douglas Carter Beane)

The other reviews...well, it's really a simple story - they all seem to have been built on the same pattern (it's quite impressive - and always disconcerting - to see such unanimity...) - a pattern which can be summarized as : Laughs are there but a plot is missing (The Washington examiner)

First part of the reviews: the set is great, the actors are good...

Romping about designer Allen Moyer's ultra-glam setting, complete with a spiral staircase and a grand piano, an aristocratic Lithgow and a bewitching Ehle do their considerable best to evoke Scott and Zelda, Noel and Gertie and the Lunts are rolled into one. (newjerseynewsroom.com)

John Lithgow, an actor who can play comedy with a heavy hand and make it seem
extraordinarily right, is the former [Mr Fitch]; a lower-tier newspaper gossiper feeling pangs of frustration for wasting his life on a trashy column instead of
writing that novel lingering in his imagination. Jennifer Ehle has an endearing quality as the Jersey girl who fell in love with Manhattan, and conveniently, with one who makes his living surveying its nightlife. (broadwayworld.com)

Lithgow and Ehle are working hard to be amusing. (curtainup.com)

...But, second part of the reviews, the play, stuffed with bons mots, epigrams, inside jokes, social and intellectual references and arch put-downs, falls to rise interest during two hours:

It doesn't take long before their repartee sounds more like a debate in which the content is only as good as the speed with which it is delivered. (curtainup.com)

What might have been mildly amusing as a one-act eventually gets strained to the breaking point, and a plot element involving the pair's making up a fictional character who suddenly takes on a life of his own is far too silly to make its intended satirical point (reuters.com)

Quantity has taken over quality in Beane's latest play...(...) The barrage of
self-satisfied quips is so relentless that the show quickly becomes numbing (...) It doesn't make sense, like anything else in this bewildering, sloppily plotted comedy. (nypost.com)

I certainly forgot other reviews of this type - sorry for their authors, but they're all quite the same! - the meanest one being Joe Dziemianowicz's at NY Daily News.

More original ones includes Cindy Adams review with 10 reasons why the Fitches' are not real journalists - I thought that was obvious, given it's theatre...- and, more funny, how it could be fixed.

An article well informed on all the references (87 !!) mentioned in the play. You can complete it if they are some missing.