I'm tired, so I reserve the right to change my mind after I've mulled over the play and after I've had some sleep. Also, forgive any errors. My punctuation and grammar aren't the best after midnight.. Also, there are clearly some spoilers ahead.
First things first, as expected both Cumberbatch and Miller are brilliant. They are well-matched in this. Karl Johnson as De Lacey and Naomie Harris as Elizabeth were both stand-outs in the rest of the cast. The only cast member who I think did anything approaching a poor job was George Harris as Victor's father. I have to be honest here, I was distracted by his Caribbean accent, but I feel if he'd been doing a better job with the performance I might not have found it so distracting.
I had expected to have some issues with the set design, but I was pleasantly surprised to find that "I ain't bovered." It was sparse. It was also tilted in a couple of scenes as some of the sets corkscrew up from underneath the stage. It was a little odd to see the actors work on a slope like that. I think the sparsity of the sets was intended so the audience would really focus on the actors, and it was mostly successful in that regard. Really the only sets that failed for me were the tilted bedroom and drawing room sets where the sloping floors really took me out of the moment.
They spent far too much time on the Creature learning how to move and walk but not nearly enough time showing Victor's reaction to the Creature and why he abandoned him. It would have been nice if they had borrowed five minutes from the "birth" scene and devoted it to the initial interaction between Victor and his creation.
Surprisingly, I feel the writing was probably the weakest point of the play. There are moments of brilliance, but they're mostly provided by John Milton quotes. There were some lovely humorous moments, but some of the humor fell flat. I guess I expected more than a cheap laugh over Scottish accents and the Creature's first words being "piss off.". Also, at one point Elizabeth prays to God to help Victor and it was so painfully awkward and cheesy that I had to stifle an uncomfortable giggle. All I could think of was "Are You There God? It's Me, Margaret." It was meant to be earnest and demonstrate Elizabeth's goodness and her love for Victor; it didn't quite come off that way. There was more than one moment like that which was meant to be serious and earnest but instead came across as kind of corny. Also, I think they could have better explored why Elizabeth was so devoted to Victor as to put up with his shoddy treatment of her. I know she was supposed to love him, but that's just taken as a given, and it's never really explained or shown as to why she's so in love with him. It wasn't even clear to me whether Victor returned that love. He says he does at one point, but their whole relationship seemed very vague and amorphous, and I don't think that was intentional.
All that having been said, it really was a very good production. Other than the initial scene, the pacing was excellent. A good bit of the humor did hit the mark, and I appreciate that they tried to inject a fair amount of humor into the play. Also, I loved the way you could see the influence of having played the Creature on JLM's performance as Victor. You could see some of the Creature's spastic gestures and speech patterns very evident in Victor. The rape and murder scene of Elizabeth was handled as tastefully as possible, and that scene and its aftermath were truly touching. Obviously the truly brilliant moments were the scenes between BC and JLM. They play off of each other beautifully, and the caliber of their performances reached it's pinnacle when they were on the stage together. The final scene was absolutely heart-wrenching.
On a final note, it was so good I'm definitely going to go see it again with BC as Victor and JLM as the Creature, and not just out of fangirlishness (it's a word now). As we left the theatre, my sister, who is not a fangirl of Sherlock or BC nor a particular fan of the theatre, said she wants to see the other version with the roles reversed. That's how good it is.