As a kid who could only play videogames on weekends, it took me years to finish this game. More accurately, it took me years to stop procrastinating long enough to play it all the way through, so I have no idea how long it actually took me, but it probably took many hours of my life to finish the game. The world is fascinating, fun and creepy. There are bright and dark levels, there are easy and tricky villains. The little sprites you have to rescue can get annoying because they constantly whine until you save them, and then they give you advice that most people figured out already.
The worlds are very creative, and unlike the cheaper games of the Gamecube era (ones based off movies, like Chicken Little or The Incredibles 2 come to mind) the style takes advantage of what it has. In many games, you can see how they had to make a character's head a certain shape that doesn't look right, and the colors and details are flat. Warioworld is beautiful and you can tell the developers took their time on it.
The games aren't too easy, but not too hard. For every good game there seems to be one level you get stuck on, and I think for this game it was either the boss with the creepy green guy or the clown who could take off his head. There are little things and treasures that you don't have to collect to pass the levels, but when you collect all the statue bits Wario gets another half-heart (more health) so it's good to do that. Collecting everything is what I consider the hardest part in the game. You might have to go through the levels two or three (or in my case, about four) times to find every foot and mustache for the statue and every treasure.
The last battle is disappointing, from what I remember. Not that exciting of an ending. But I would still recommend this game to anyone who likes Mario or Wario games, or just weird games.