Today I managed to 100% complete RTDL. It was an amazing experience but I have to admit that I really wanted the game to end. The boss battles in Extra Mode are actually much harder and required a few tries, except for the giant tree in the first world but other than that the game was VERY cool. The coop gameplay is great though a bit uncomfortable for the non-Kirby players who get a game over every time the hero dies.
I didn't get the gold medals in the challenge rooms and I don't think I ever will. In fact, I won't get back to this game in a long time. Unless I have to show it to somebody, but I loved it and it's definitely worth a try. Games like this really make me be sure that I'm a fan of the series. Now I still have to complete Kirby's Adventure, Mass Attack and Amazing Mirror so there's still a lot of Kirby stuff to come.