Picked it up this morning and wrapped up World 1. So far I love the level designs and the interaction with foreground and background (especially in the sunset level) shows a lot of creativity. I love being able to just quit and it saves your progress instead of worrying about spending coins.
If I were to nit-pick, it would only be two things so far. For whatever reason, I just cannot get used to the motion controls. I'm messing up roll jumps, and there is a delay between shaking the wiimote and performing the move. It's pretty frustrating when you're used to playing with precision platformers. I've been going back and forth between wiimote & nunchuk and wiimote on the side. So far, I've been enjoying the wiimote on it's side approach, but I'm going to try out the other control scheme a couple of more times before ruling it out.
My only other gripe would be that the mine-cart levels are one hit kills. At least in DKC2, (can't remember the original) if you had Diddy and Dixie together and hit an obstacle, you'd lose your partner but you'd still have another chance. Here, if you have DK and Diddy with four hearts, one mess up will send you back to the checkpoint, which can get frustrating pretty fast.
Hopefully the motion controls will click after a little more time with the game. I was just hoping by the end of the first world I'd be good to go. Regardless, I know this post sounds pretty negative, but even with the issues I've mentioned above, I am having a ton of fun with the game.
edit: Also noticed when rebooting, it saves your lives and banana coins. Even though it should be a given, that always annoyed me in the recent Mario games when you would start back at 5 lives even though you had collected 20.