BakerWilliams said: "Terry Teachout is mixed/negative on the first 3/4 and really loves the last quarter (I must say, I'm in agreement.)
https://www.wsj.com/articles/significant-other-review-coming-of-age-1488504600?tesla=y
Since I know there is a paywall, here's a single passage from the review:
Since “Significant Other” made it well past intermission without showing any signs of becoming unpredictable, it seemed safe to assume that it would remain so to the finish line. But halfway through the second act, it suddenly metamorphoses into a different play with the same characters, a dead-serious look at the problem of being a lonely singleton in a world full of hurtfully contented couples. The gears shift when Jordan confronts Laura at her bachelorette party, angrily telling her that she has deserted him for her fiancé and that “your wedding is my funeral….it somehow enshrines the officially non-existent role I’ll play in your life from now on except as occasional court jester and pitiable reminder of what happens to people who never find someone.” Yes, it’s unfair, but Jordan believes it, and his desperation is so palpable that you can’t help but sympathize—as does Laura, who never saw it coming. From this moment on, the cliché tap is shut off and every character in “Significant Other” becomes touchingly real, the way they should have been all along. Nor does Mr. Harmon, to his infinite credit, cheat the audience at evening’s end: I mustn’t give away the curtain, but suffice it to say that what happens (or, rather, doesn’t happen) is powerfully true to the mature sense of life’s limitations that Jordan has acquired at long last."
Two years after missing the Broadway production of this show because we missed our flight, I finally saw Significant Other in a regional production at the San Francisco Playhouse. I often disagree with Terry Teachout, but I think he was dead-on here. I basically didn't like the play for the first three-quarters. Jordan was annoyingly needy, the female friends (save Laura) aren't terribly funny or interesting, the grandmother scenes were fine but felt stuck in there to humanize him.
But it's all building to the big blowup between Jordan and Laura at her bachelorette party, and that's when all the annoying stuff melts away and the play suddenly became compelling. Kyle Cameron, who plays Jordan, and Ruibo Qian as the blindsided Laura, are terrific in that long scene in which all of Jordan's grievances (both petty and legitimate) are unleashed on his best friend. It's such a good scene, in part because Jordan's generalized upset is targeted at a specific person, and he attacks her in a way that only someone who knows her well can do. It's unfair to Laura, and she lashes back, but the play suddenly has weight. That carries through the remainder of the play, which earns its semi-happy ending.