It's a game that has a little something for everyone, all without compromising its unyielding, unique, and undivided attention on its mission to blast pure adorableness into the world in all directions. If Kirby and the Rainbow Curse doesn't make you smile, you may need to see a doctor for that.
Like the Metroid titles that inspired it, the music, graphics, and world/enemy design are more than worth revisiting a few times a year. This may be Renegade Kid's best game yet, despite its relatively short length. I hope this isn't the last time we'll be invited to adventure with this mysterious new space explorer. Were they secretly a girl the whole time? Hopefully we'll find out next mission.
There are plenty of irreverent characters to talk to, fetch quests to give a limited sense of purpose/achievement, and a nice split between breezy and tough areas, but all the same, you'll likely want to take more frequent breaks than in chapters one and two. Interest will dry out more quickly, especially if you focus on the story, which is either trying to be "so stupid that it becomes awesome," is designed to make you hate stories in games, or both. Either way, Knytt Underground is brilliantly-designed 2D platformer that offers a ton of content. Just don't think about the story too much. "blah blah blah blah blah" indeed.