I really liked the play!  It had its comic moments, but as the play progressed the story was more ironic than "ha ha" funny, and it doesn't really have a happy ending.  Actually, I think the ending kind of leaves things up in the air, and I'm not sure whether that was the playwright's intent, or whether I just didn't understand some of the subtle implications in the dialogue, or whether it just still needs some work.  I do agree with the Hartford Courant critic in the review I sent you that the Second Act seems a little anti-climactic after the tension and release which occur in the First Act.
Basically what I got out of it was an indictment of the class system which ruled the behavior of all these characters, and the misery it could cause.     

Frank was at first quite funny (as in "ha ha"), prancing around the stage and spouting all these silly French phrases, but then with great subtlety he turned into a very cruel and sadistic boor, just as the promos described him.  It really was fascinating to see him effect this transition!!  By the end of the play his character was someone you pretty much loved to hate, but at the same time I think Frank's characterization contained an undercurrent of insecurity about his status in society, and a resulting need to make someone look inferior in order to appear superior by comparison.

You asked about what was the "best part", and for me it was watching Frank do all kinds of "business" during the First Act, while Alan Bates's character Vassily was telling a very long-winded story about his lost inheritance.  Frank was in turn eating voraciously (during which you could still understand every syllable he uttered!), keeping everyone's glasses filled with champagne, unbuttoning his vest, rebuttoning his vest, strutting around,........  And during all this activity, he was doing that transition from seemingly sympathetic supporter of  Vassily, to someone who was just trying to make him look the fool!!

Alan Bates was also wonderful.  He's a very small guy, but when he's holding forth he positively fills up a stage!  He had about a ten-minute soliloquy in the Second Act, and you literally could have heard a pin drop in the house -- I don't think I even heard a soul cough for a good ten minutes!!  That was another "best part" for me!

In spite of being a little confused by the ending, I'm very interested in seeing another performance later on when it's on Broadway, to see how much it will have changed between now and then.  I used to do some backstage work in community theatre several years ago, and even just hearing a play from backstage, no two performances of any play were ever the same.  That's one of the marvelous and exciting things about live theatre!!  So I look forward to seeing that principle in action with this show!  And of course, I'm especially interested in seeing how Frank's performance will have evolved, since we know his philosophy is to never try to duplicate a past performance, and to always endeavor to keep it fresh!