As a quick note before I start off, Kirby's Epic Yarn shares few similarities to other Kirby games, so don't be put off if you haven't played others in the series. I haven't either, and I didn't feel left out or lost at all.
As previously stated, this game is set in yarn world - Kirby is made out of yarn too, and his abilities reflect that. He is able to change shape depending on the circumstances (submarine in water, car for a speed boost, parachute when falling), which adds a lot of variety to the worlds. Bonus levels and certain stages of regular levels transform him into special shapes, like UFOs or dolphins. These were some of my favorite parts. The developers did a great job integrating lots of variety into the game play in this regard, adding enough to mix things up periodically without overdoing it.
Dolphin Kirby swimming through rings
Kirby's main ability in his regular form (besides jumping, of course) is whipping out a yarn lasso of sorts. This lasso can be used to rope enemies (resulting in them falling into bits of yarn) or interact with buttons or patches in the world, either to allow you to get somewhere you couldn't otherwise, or reveal secret collectibles in the level. I felt like the developers did a good job integrating this mechanic as well.
Kirby pulling a button to stop the lava bursts.
Speaking of collectibles, this game has its fair share. Each level has two pieces of furniture for your kickin' house (which you can trick out in between levels if you so desire) and one CD that unlocks music from the game. Every level has a plethora of beads (dropped by enemies, hidden in hard to reach locations, and just sitting out for the taking) that if you collect enough give you bronze, silver, or gold medals at the end of the level. Additionally, throughout each level there are extra collectible pieces to a spin wheel that let you have a chance of getting extra beads at the end of the level, which is nice if you're prone to screwing up over and over again.
The beads and medals don't actually matter a whole lot except on the boss battles, where if you get enough gems you get an extra patch to unlock two bonus levels in each world. Other than that, the only purpose they serve is to act as currency for an in-game shop to buy more stuff for Kirby's house. Still, it's very satisfying to get gold medals.
I played through this whole game co-op, which alternated between being helpful an frustrating. Each player takes up his/her own space, so you can imagine there are plenty of opportunities to bump each other off the edge. Both players also share beads, so when one person screws up and dies, you're both punished. Still, there were also plenty of times where we'd give each other a boost to get to a particularly tricky part of the map for a precious collectible. I enjoyed playing with my husband, but I think he sometimes got frustrated when I got hung up on some of the platforming. :) All levels are beatable single player regardless.
This boss was particularly obnoxious co-op.
Difficulty-wise, this game isn't really that hard. Both the art-style and difficulty lead me to believe it was developed to be kid-friendly. True, it takes a bit of skill to gold medal every level and get all the collectibles, but strictly as far as beating it goes, I think most kids ages 8+ (maybe even younger) could get through the majority of the levels without too much trouble. It has liberal checkpoints if you die, the only penalty being that you lose most of your beads, and the enemies don't instantly kill you if you touch them (a nice change). Instead, they cause you to spill a lot of your beads if you come in contact with their attack, which you can then recover if you're quick (there's about a 15 second window before they start disappearing). Kirby can roll up enemy yarn remains with his lasso too and use the resulting yarn ball as a weapon to chuck at other enemies. If you play co-op, you can also roll up your partner in a yarn ball and toss it at enemies to attack them.
Pictured: submarine Kirby holding a yarn ball, preparing to chuck it at some poor defenseless fish.
This game is not without its flaws (mostly in the co-op), but overall it's a fun game and worth the $20 we paid for it. I probably wouldn't pay more than that, since it is pretty short, but if you have kids and/or are looking for an easy, whimsical Wii game, it's worth playing through.