This game is the reason I bought a GameCube. I am a big Mario fan, so naturally I liked this game too.
This game is a really fun 3D platformer, and the water pump, FLUDD, adds a whole new dimension to the game. If you have played Mario 64, you know what you're expected to do: Go though each "episode" of each level whose objective is to collect the star (shines they are called now). But you'll probably miss the classic elements of Mario's world, like the wierd landscapes, castles, snow areas, goombas, koopas, etc.
Instead, all the levels in the game take place on the same tropical island, quite a change from the variety of past Mario games. Though the levels aren't completely different from each other in terms of theme, you can still expect them to be varied enough. For example, one level takes place in a village surrounded by hills on the centre of the island, while another takes place in a ship-filled harbor on the coast. But also unlike past Mario games, these area are populated with generic and uninspired characters and enemies.
The lack of all these don't actually detract from the gameplay, but still, there are certain things you'd expect in a Mario game. But at least you can expect the same great platforming gameplay.