It's mediocre. I liked it's art style, and some of the puzzles are alright. However, there are a number of sections in the game where the controls can't quite reliably match what's expected of you that can cause some frustration. You never control the girl directly; you 'guide' her with the green firefly which is awkward until you get used to it.
The default touchscreen controls work for the first few areas, but after that the game starts expecting quicker, precise movement that's tricky for the touchscreen. Changing to analog stick controls makes it a lot more feasible/bearable, but it's still not perfect as the fireflies' movement is wonky w/ analog sticks. Briefly tapping the stick doesn't always consistently move the same distance, which is infuriating during a couple parts where you navigate through a electrified maze that's only twice as wide as the firefly. Also, the 4th boss is a total crapshoot that largely comes down to luck (or clever usage of an external camera) to beat.
The majority of the game is actually kind of easy with analog controls; it's just that the few parts that are extremely frustrating really stick out and soured my experience with the game.