Kirby's Epic Yarn is one of the most charming games I have ever played, though it certainly is one of those games where enjoyment comes from the sights and sounds and not from the challenge.
While most characters and enemies have simple yarn-based designs, they are functional and mostly fun to look at. I also think they are very well animated, moving about in a playful, childish manner, meaning there isn't really anything in this game that comes across as scary or imposing - everything is designed to be cute. Additionally, the levels themselves are absolutely gorgeous - to simply enjoy the sights of this game is one of the main reasons to play it. It should also be noted that this game has a stellar soundtrack, complementing each stage with cute and playful (mostly piano-based) tunes that really add to the charm of each stage.
Kirby's Epic Yarn is doubtlessly fun to play: It controls very well and creates variety by having Kirby transform into different shapes: instead of running he changes his shape to a small car, and instead of gliding through the air he transforms into a parachute. Some levels force you into a very specific shape - a dolphin, a firetruck, a tank, a surfer, and even space ship. Indeed, near the end of the game there were a couple of stages playing like classical horizontal and vertical spaceship shoot-em-ups, which I enjoyed a lot. This game also allows for 2 player shared-screen co-op which is very fun. This, quite interestingly, means that some transformations are made so that one player controls certain functions (such as movement, thrust, or jumping) while the other character controls extra functions like shooting, extra jumping, or turbo. It makes for a nice team-building exercise.
One thing that might be seen as a strength and a weakness is that there are no '1ups' in this game - it simply is not possible to die. When you get hit or fall down a hole you lose gems collected throughout the stage, and these gems affect what kind of completion-medals you are awarded at the end of the stage, but you are never thrown off the stage. It is, thus, perfectly possible to reach the end of each stage even if you are hit with everything thrown at you. Completing boss fights certainly requires you to grasp a few simple mechanics to complete, but no level will actually kill you. This makes the game perfect for casual players or beginners - I had much fun playing this with my GF and can imagine using this to teach young children the ropes of 2D platforming - but may leave more advanced players disappointed. Even the extra challenge of collecting the three collectables and carrying enough gems at the end to get a gold medal on every level isn't particularly difficult.
Like most 2D platformers, Kirby's Epic Yarn has a cute and 'functional' story involving an evil Yarn-based magician trying to transform the world and everyone in it into yarn. It plays much like a story book - complete with narrator and all - pushing the story forward a bit after each completed 'world'. It's cute and very child-friendly, and that's really all it tries to be.
Personally I had a lot of fun with Kirby's Epic Yarn - it controls great and there are so many great sights and sounds to enjoy along the way. As a fan of the horizontal and vertical shooter genres I particularly enjoyed those transformation stages. The lack of really challenge did not really bother me - it made Kirby's Epic Yarn a great co-op experience and--importantly--the lack of mechanical hurdles meant that I wasn't too distracted to enjoy the sights and sounds. It was a relaxing game to play, no doubt about it.
I give Kirby's Epic Yarn an 8,5 out of 10 - it deserves high praise for its presentation, control scheme, and co-op friendly setup, but I do think it could have added an extra, optional layer of real challenge for more advanced players.