I've furiously avoiding spoilers but I've allowed myself the listening of the OST. I agree pretty much with your assessment: pleasantly surprised. Having said that, it hasn't blown me away, and I cannot rate the new vocalist terribly highly. That isn't a fault on her, Persona has always been weakest when it comes to vocal tracks. I only really thought Lotus Juice's work on Persona 3 to be the best vocal tracks. Persona 4 had some good vocals.

I keep listening to 'Last Surprise', the standard battle music in this game, and while it is good, it weakens somewhat when Lyn starts singing. It's unfortunate. Having said that, many of the instrumentals are really very good. Persona's tracks have always been best listened to 'in-game' anyway.