I also caught the second preview tonight, overall there was a lot I liked about it.
While I haven't seen previous versions, I got a kick out of what appeared to me being Aaron Sorkin rewriting Merlyn to be a very Jed Bartlet like figure. The smartest guy in the room, constantly trying to take the piss other officials, in this case Arthur and the knights.
I thought Phillipa Soo was fabulous. This is the fourth musical I've seen her in (after Hamilton, Amélie, and Into The Woods) and she's never really connected for me as a performer before. Maybe that's not really fair because Eliza Schuyler is an underwritten part and Amélie just wasn't good, but anyway I thought Soo was really charming and nailed the Julie Andrews style vocals.
I also loved the Arthur/Geunevere meet-cute, thought how Bartlett Sher used shadows in the set was really cool, and thought both Burnap and Donica were well cast as the other leads.
Not as great, the show didn't end until 11:11pm and more specifically the second half of the first act really drags. For instance, when Lancelot gets knighted, Arthur knights 3 or 4 other people first, which serves no purpose what so ever in pushing the story forward. There's a lot of fat along those lines that can be trimmed.
Additionally, a lot about the central love triangle just confused me. Did Arthur and Geunevere ever have sex in their marriage? Arthur eventually reveals he's been concealing his feelings because of the power misbalance in their relationship. If that bothered him, he would likely view sex as an ever bigger consent issue.
If sex was a component missing from Geunevere's marriage, that might explain her intense attraction to Lancelot a little more clearly. In the first act, I kept expecting to learn that they had been secret lovers before she was promised to Arthur, and they broke things off for the good of French-English relations. But no, as written it's just lust at first sight.