Just finished world 5. What a fantastic game. So many memorable levels and visual elements. Spingy Spores has such a great setting and Muncher Marathon (the spider level) has to be an all-time classic.
Mangoruby was quite a bastard. I felt very satisfied when I had beaten him but it took me a number of tries to get a feel for the fight. Also, the fight was one of a number of moments that made me wish I was playing this on my TV. The encounter just felt too cramped for the 3DS. I think I'm going to enjoying doing the time attack medals for this one though (whenever I get around to going for those).
I'm really enjoying my progression through the game. Sometimes I'll sit down and want to play a new level, other times I'll want to go back and try to collect stuff on the earlier levels. The K-O-N-G letters are always essentially fair, so I'll always go for those on my first return. The puzzle pieces can be more tricky but, by the time I have spent enough time with a level to first beat it and then possibly returned to get the letters, I usually will have gotten most or all of the pieces. From there it is easy enough to hire Squawks to get the last 1 or 2.
Also, I really like the unlockable galleries, similar to those in the Prime series. They add an extra bit of motivation to the task of going back to complete earlier levels. I have only unlocked the 'Diorama' for the first world so far (achieved by collecting all letter and all puzzle pieces) but that was a great feeling. There is just so much content here. I feel like I will be playing this for another 2 months.