I saw this tonight. Was hopeful coming in, but unfortunately, the show is a bit of a mess.
The tone of the show is trying to imply "this is a fun romp, don't take this too seriously", but there's so many plot threads (Vera being upstaged by Iceboy, her relationship with her boyfriend, the secondary romance with Iceboy and the maid, O'Neill being pulled out of his writer's block, a butler who is Frankenstein for most of the show, to name a few), that you can't help but take in the show critically, and it just comes off feeling overstuffed. It's no surprise that the show has been tinkered with and developed on and off for a quarter of a century.
Nick Offerman was the best actor. He does his usual schtick of dry-wit curmudgeon, but it works well for O'Neill, and he gets some of the best lines of the night (like when he tells everyone in the audience, including the children, to get liquored up at intermission). Megan Mullally is really funny, but she leans too far into her character voice (very similar to Elizabeth in Young Frankenstein, as others have pointed out), to the point where some of her lines were near-unintelligible.
The music was a big disappointment. There's no real musical identity, unlike Urinetown, which captures a Kurt Weill pastiche brilliantly. There's 30s jazz, modern pop, Grey Henson gets a Menken-esque ballad. Most of the songs have really clunky lyrics, with way too many syllables being crammed into way too little melody.
It's not impossible that this show gets to Broadway, but it'll need a substantial amount of work, primarily in stripping away a lot of the plot and just letting this show be the silly diversion that it wants to be.