W00T!
The best way I can describe this is as an upper echelon flash game with some serious replay value. I honestly couldn't think of a better game to introduce the Newgrounds Metal system with. (Which is an absolutely awesome feature, by the way).
The art has that perfect balance of happy-go-lucky cartoon style and pure carnage and gore. I would really love to see more games with that "in between" vibe. It appeals to both a younger and older audience quite well, IMO.
The gameplay was perfect. the enemies were easy to engage, yet took a while to defeat. The gameplay was also good enough to the point where you can start to incorperate your own strategy into beating enemies and bosses. (Like tricking the KK into being bombed by BFK's yarn-balls, or using enemies long-range attacks to injure their partners, for example).
The audio this game showcased fit like a glove, all the way down to the tempo of the fighting, and the pace at which the characters move. The music matches it all perfectly! The quality of the audio is right up to par with the graphics which makes for a really well-rounded game with no weak spots.
The difficulty is really spot-on, for me anyway. Getting far into the game wasn't too challenging, but actually beating it, on the other hand, was. Too many games are either too easy and get boring fast, or are too hard and are simply given up on. This game offered just the right amount of challenge to keep you interested and coming back for more.
Aside from featuring the all-new Metals feature, the addition of mini games kept the game interesting and acted as nice "intermissions" between levels. Adding unlockable hidden characters also added to that extra dimension, not to mention made for good replay value.
Overall, I'll simply put this as a top-notch game. To be put up there as one of the best ever submitted to Newgrounds. Simply astounding work!