Tldr: One of the best games I played of all time.
This game is really well made, you have awesome character development, an extremly fitting art style and background music which makes you feel comfortable and the map of Knytt Underground is huge and got tons of secrets.
The gameplay mechanic is innovative and after you got a hang of the controls you can involve your surroundings in your gameplay.
Completing the main story of the game will take you about 6 hours, but you really want to miss out all the sidequests, additional characters, secret challenges, several ending and and and.

If you want to have an atmospheric and relaxing experience, try out Knytt, you will not regret it.