Got about 70% completion. It took me a little over 10 hours to beat.
I've never seriously played a DKC game before (I played one of the SNES DKCs (can't remember which) on breaks a few times when it was the demo game at the Toys "R" Us I worked at, and didn't really like it back then).
There is a nice mix of stage types, and I enjoyed most of them (even the underwater stages were fun). I do like a good mine cart stage, and this had a couple. Being able to control either Diddy or Donkey Kong could have been a cool feature, except that there is almost no difference between the two, other than Donkey can take out a few enemies that Diddy can't and Donkey has the ground slap which has very little use outside of uncovering some secret items for collection purposes. It really just turns into having a second hit point (or third if you're riding an animal buddy). The animal buddies are also a neat thing, it's just too bad that they aren't in many stages (Engarde being the most useful, IMO). I also like how it tells you how many of the items/secrets you've gotten in each stage, which, for me, would make it worthwhile to try to 100% the game since I know exactly where I would need to hunt down the items I missed.
Overall it's a pretty easy game. Only a handful of stages really gave me any real challenge. The bosses range from easy to extremely easy. Because of how easy it is to rack up lives, especially once you get to the first Dance stage, I have to wonder what exactly the point of lives is in this game. I spent pretty much the entire game with 30-40 lives. The bonus areas are a nice break, but again, they make racking up a bunch of lives very easy, especially when you can just about instantly reenter most of them.
Outside of the difficulty, the only real issues I have with the game are:
1) your hitboxes being larger than the characters' sprites (although for King K.Rool, his is smaller than his sprite, so I actually jumped through him a few times)
2) the "platform detection" (for lack of the proper term) where I would fall to my death while watching them go through the solid platform, or that you can be bouncing on the tires when you are so far to the left or right that you aren't even touching the tire.
I guess they added a couple extra features for the GBA release:
1) DK Attack, where you can play a stage (on a timer) and score points to set high scores.
2) The two mini-games found in-game, Fishing and Dance, where you also set high scores.
