Of the MegaTen games I've played, this is my favorite. SMT4 had more of an impact on me personally, but I like this one more overall. The Schwarzwelt is a great setting. I like seeing a JRPG with an adult cast, that know what they're doing. I like the crew fight amongst themselves, it adds real drama to the story. Speaking of said story, it's very good. I feel like the more you look into it, the more you appreciate it, and the more you really start to think (and I'll leave it at that for now).
The combat is great. In this game, if you use demons of the same alignment (including your alignment), you'll get extra hits in if you hit an opponent's weak spot. I love this system because it makes party construction more interesting, since it gives you something you can build around, and makes you use demons you wouldn't normally use. That being said, I hate the stone status condition.
Dungeon crawling is fun. There are some annoying parts but overall I like it.
The game is hard, for sure, but I really think Atlus nailed it here.
As always, demon collecting/fusion is fun and I learning about each and every one of them. However, demon negotiation is WAYYYY better in this game. It's more focused on learning the demons personality and picking up on patterns, whereas in a lot of other MegaTen games it's mostly RNG.
This game's OST is also spectacular.