So firstly, good job. You took up about two hours of my time in collecting all 46 of the mangoes; some of those got tricky!
Firstly, I really like the original characters. While their names are a little strange (I've never met somebody called 'Yeah', before), they each had their own abilities, style and voices, which made them all feel unique, inadvertently giving them each of their own personalities, which was very nice. However, one issue I did have with the characters was the very limited amount of soundclips there were - the amount of times I heard "Killer!" and "Boss!" is enough to drive any person insane.
Another problem I had with the characters was that none of them actually felt comfortable to play as. Yeah walked far too slow to use frequently, Jam slipped and slid all over the place, and the least said about Fury's agonizing walking pattern the better.
Having said that, the level design WAS incredibly clever, and the varying background designs were more than enough to mix things up with each level.
For me, though, the highlight was most definitely the highlight. Within the factory-based stages, the music was very much reminiscent to the pirate ship sections in The Legend of Zelda: Wind Waker and Lanayru Mines sections in The Legend of Zelda: Skyward Sword. To have music quality that of Nintendo's is a huge honour! And I defy ANYBODY who states they didn't laugh at 'What's Up' music featuring in one of the final levels.
In conclusion, the game was very well made, the characters were fantastic, though control was a little iffy and some of the level design was a tad repetitive. Having said that, I can't wait to see more from Yeah, Jam and Fury! Killer!