I have to say, I'm really impressed with myself. Beat this game 100% just the other day, after owning it since it first came out. I initially gave up trying to complete World 5's Dark World (and thus The End's Dark World and most of Cotton Alley) because I got too frustrated. The game is undoubtedly brutal. However, recently I had a spark of inspiration to go back and try to complete the game, because leaving it as it was bugged me. Besides, I needed a good challenge.
I still encountered a weird glitch where the game gave me the achievement (X360 version) for 100% completion even though I hadn't even completed all of Cotton Alley. Additionally, my completion rate is actually 107% and I don't really know why. I also collected a bandage in Cotton Alley's Dark World, for 101 bandages overall.
EDIT: Reading some of the thread, I guess that's not a glitch?
Oh, and I went for all A+ times as well and succeeded.
I have not downloaded/completed all of "Teh Internets" but I actually don't consider that to be part of the main game. And honestly, I don't even want to bother. At least, not yet. I need to cool down for a while after beating the non-DLC chapters of the game.
This game is a true test of patience and platforming skill. But it has the capability of driving the player mad. At one point I was thinking of checking myself into a mental institution. Hah. Additionally, it really promotes masochism more than any other game in recent memory.