Another musical theatre Star Trek fan who finally caught up with the episode. Obviously I loved it. The writers took their cue from "Once More With Feelings" and used the "very special episode" conceit to progress seasonal character arcs (as they did with "Those Old Scientists"). You could tell was the intent even from the "previously on" recap with a bunch of relationship tensions ready to be revealed through song. (The bunnies reference was a nice nod to the Buffy episode.)
I knew Celia Rose Gooding could sing (although, sadly, she was off when I saw Jagged Little Pill on Broadway), so the actor whose vocal chops surprised me most was Christina Chong. I see from her wikipedia entry that she was actually in the Elton John musical Aida in Berlin, so that makes sense now.
Maybe my favourite minor running gag was how the characters always heard and acknowledged the backing music - in dialogue or with just a glance. I could go on a pretentious detour on mimetic vs diegetic music, but won't.
But I wasn't blind to some of the episode's flaws either. The biggest to me was that the songs lacked the craft and polish of really good musical theatre songs, with (for instance) many imperfect rhymes and awkward prosody (putting the stress on the wrong syl-LA-ble of a word). Crazy Ex-Girlfriend, a show that I loved, suffered from the same issue.
A minor complaint is that I didn't think we need the rules of musical theatre to be so explicitly lampshaded by the characters, although La'an treating it as security (and personal, emotional) risk was cute - and in character.