There's some things in Ys that makes the series quite special. I may like some Ys games more than others, but I do sill like all of them. That is because each game has unique aspects to them while still maintaining the successful formula that let the series live for more than 20 years.
Ys IX: Monstrum Nox is not any different. It has everything you'd expect from a Ys game but it's also an unique addition to the franchise. The urban fantasy setting is quite refreshing and Balduq do feel like a big and relevant city. There'll be a lot of urban scenarios to explore (And lots of collectibles too!) and it hardly gets tiring since the city has lots of different places. You'll still get to see some greenery like always, but it'll take time until you get there.
Gameplay wise the game follows pretty close to what Ys VIII did. There is some new things like Monstrum's gifts, which is a lot of fun, but if you've played Ys VIII you'll have a very similar experience. The game may feel a bit on the easy side though, but there's a lot of difficult levels for you if you want to have a challenging time.
Story is great! It does know how to balance story progression, character development and exploration while still bringing some really interesting mysteries that makes you think. Though I do recommend you to play until chapter 4 before you start exploring and getting the collectibles, it may make your life easier.
The characters are really cool! It's easy to like their personalities after you go through their respective character arcs and their designs are awesome. They also feels good to play with, so it's always a good time. There's a great synergy between all playable characters that makes their interactions great. The side characters are all quite memorable too, of course.
Music-wise it's the same as other Ys games, which is to say it's fantastic. Really adds a lot to the atmosphere of each place. A shame we didn't get an opening though.
Now then, if you read thus far: Go play it! Ys fans will probably love the game. If you're new to the series, you may start with this one, no problem (I do recommend starting from the first one, though)! My opinion may be a bit on the biased side since I'm a big fan of this series, but I believe Ys IX truly deserves the praise it gets. It's a great game! Now to wait until Falcom releases Ys X.