One of the best platform games Nintendo has made. There's just so much going on here that it will keep you busy for a long time.
There's lots of different worlds, with lots of stages in each world. The last stage you'll have to fight a boss creature in order to move on. Also challenging is that in order to unlock the extra stage in each world, you need to get 100% in each of the other stages. That means you have to do a lot of searching for red coins, stars and flowers. The flowers are often hidden in strange places. The red coins are gotten by collecting a lot of yellow coins or by hitting different characters or items. You can collect lots of stars, but every time you get hit and baby Mario goes flying off, your stars are depleted. So if you have all the stars right before the end of the level, and you get hit, you need to find a few more stars in order to get 100%. This is quite challenging.
There's also a lot of minigames that you can play for power ups or extra lives.
Overall this is the best of the Mario Advance games, in my opinion. Although you're not playing as Mario, it's still a great game with a lot of depth and replay value.