I finished Donkey Kong Country Returns today. By 'finished' I mean beat the final boss. There is still secret level to unlock (more on that later). Overall, I'd say I enjoyed the game, but I definitely have complaints.
My first complaint is regarding the difficulty. It seems to be aligned along the path of "This is hard, and not in the 'Oh! I figured it out!' sort of way, but we're going to give you plenty of lives to deal with it." This means that you'll often get stuck in sections that have precise timing requirements and die repeatedly. I really wouldn't expect kids to beat this, they'd get frustrated far too easily.
Secondly, there are these sections where you fly a rocket barrel (no doubt borrowed from Donkey Kong Barrel Blast. There are particularly frustrating since you go from taking 2-4 hits to die to just 1. They have finicky controls and ask the player to make some fairly precision movements.
Health and DK barrels come too few and too far between in many of the more frustrating levels. You'll go for very long times without your little buddy. All the boss stages start with a DK barrel, but none of the boss battles do. So the first time you try a boss, you'll have full (4) health, but if you die and retry the boss battle you won't.
One distinct praise that I would like to have for it. Each level has 4 K.O.N.G. letters and a certain number of puzzle pieces. Collecting all of one of those groups for a stage gets you a medal. This is integral to unlocking the final super bonus level. However, you (thankfully) don't have to collect all of them. If you collect enough of them in a 'world' you unlock a secret level for that world. Beat all those secret levels and you unlock the final super bonus level. I hate having to collect all of something in a game, because some of them are particularly hard to collect. I really like having the flexibility to not collect all of them.
I'd have to say this game was enjoyable, but I it's a very tenuous enjoyment.