I didn't have a chance to watch this until last night. FWIW, I saw the Broadway revival but have never seen the 1970 film with the original off-Broadway cast. I thought the revival was really well done and was excited that the cast and Joe Mantello were brought back together to do this movie.
On the whole, I think the movie succeeds. It does a nice job of making it feel a little more expansive, what with the opening and closing sequences, plus the flashback snippets to break up the action. And even within the party itself, it includes just enough movement between different locations in the apartment to give a sense of flow. Onstage, it's really effective to have everything continuous, almost claustrophobic as you're caught within the action, but onscreen, that same approach would run the risk of looking staid. Even with some lengthy monologues, this generally feels like a movie instead of a play. That said, I thought a couple of the "expansions" lessened tension at key points, namely Alan and Harold's entrances. In both cases, I think the moments would've landed better without telegraphing their approaches.
Mostly loved the cast when I saw them onstage, and that holds true here. I actually think Jim Parsons comes across a little better here as Michael - in the show, while I thought he was good, there were moments where I could feel the acting a little more, whereas here, I feel like he inhabits the character better. He still feels a little too actory in the really big moments, like his final confrontation with Alan and his breakdown after the party, but for the most part, I buy it. For me, though, Zachary Quinto's role sticks out as feeling too "performed." I get that part of Harold's whole deal is the artifice of his carefully-crafted presentation of himself to the world, but it feels less to me like something the character is doing and more like Quinto, to borrow from Emory borrowing from Norma Desmond, going "Mr. DeMille, I'm ready for my close-up." Whereas, with Robin de Jesus as Emory, I can buy the mix of him being naturally feminine and also leaning into it deliberately as a persona, whether to scandalize "the straight" or preempt those giving him shit about his mannerisms. And obviously, we see Emory drop some of that during his telephone scene.
I don't have as much to say about Brian Hutchinson as Alan, Matt Bomer as Donald, or Charlie Carver as the Cowboy. I like some of the little bits of onscreen business adding to the Cowboy's gooberish-ness, like how he's eating cake with his hands and has frosting ALL OVER his fingers when Harold instructs him to dance. Also, how he's just standing out on the balcony when the rain starts and and the others have to bring him inside with the rest of the "presents," hee! I think with Bomer, the film medium helps Donald feel a little more present throughout - onstage, I thought he got kind of lost in the shuffle during the second half, but having reaction shots and things keeps everyone feeling involved in the proceedings.
Michael Benjamin Washington is so good as Bernard - I love the code-switching in his accent and demeanor during his telephone scene, and throughout you can feel the tension of his position in the group, the way he and Emory band together and yet simultaneously shit on each other as the two lowest on the totem pole. And Hank and Larry remain my favorites. I'll confess that I stan Andrew Rannells to a ridiculous level, and The Boys in the Band was the first time I got to see him live onstage, so more than anything, he's the reason I was thrilled that these performances were preserved for the film. He and Tuc Watkins play off of each other so well, and both Hank and Larry's telephone scenes are just splendid, although I think the shouted "IT'S FOR YOU, HANK!" worked better onstage than it does in the movie. Also, I do call shenanigans on how darkly-lit their sex scene is at the end.
Overall, I think putting these stage performance in the context of a movie adds an interesting energy. Mantello maintains the ensemble presence through the reaction shots and whatnot, but even when someone isn't in focus (or onscreen, for that matter,) it's like I can still feel them acting, contributing to the energy of the room. It comes through for me in a lot of little moments.
TL;DR - Really glad to have this film. It's a different experience than seeing it onstage, and it was fun to compare the two.