It's really, really good. I'm not sure I've ever seen lusher, more detailed interior environments. It's just staggering how good it all looks. There's rooms in the far corner of the castle, tucked away, that so many players may never see, and they look as good as the rooms in other games they lavish attention on because they know everyone will see them. It's just remarkable.
The other highlight is the combat. Magical combat in games is rare, mostly because it's just so stupid: spam buttons, spam a key over and over to win a "battle of wills" where your two colored beams meet each other to see who wants it more (there's some of that here but it's very rare and mostly a symbolic ending to some of the fights). But there's such a lovely mix of spell interactions here. Levitate them, hit them a few times, then slam them back on the ground. Freeze them, then hit them with a force spell. Set them on fire and then knock them back. Pull them closer to use a short-range spell instead. Different enemies with different weaknesses. And some have shields you can only break by matching a certain "type" of spell, based on color.
It's just shockingly fun and allows you to be really creative with which spells you equip at a given moment. I felt engaged in every little bit of combat there was, even at the end when I was able to fly around and mop up enemies in a few seconds each. Even that was fun. So, so impressed with how much thought they put into the combat.
A few downsides: the side quests are so-so. Nothing terribly special there. Some of the collectible/100%-style-options are lame, some are fun. That's kinda standard there. Personally, I'm not a 100% completionist when 100% means just running around finding things, but I actually bothered with that for several of the items here because it was just so fun to look around, period (and most involved some small level of discernment, so it was never just shoving my head in every corner). I didn't do all of them, and won't, but I did way more of that stuff than I normally bother with.
Main quest is pretty good, weaves in a lot of stuff naturally. Voice acting is superb. Writing quite good too. Just a really lovingly crafted game. Can't wait to see what they do next.