Great game overall, but it definitely feels like it was rushed.
Later, however, when I got my hands on Super Mario 64, and then Super Mario Galaxy, I realized that Sunshine has quite a few flaws. One, the game seems to lack the depth and variety of SM64 and Galaxy. Unlike the richly varied and numerous worlds and levels of 64 and Galaxy, Sunshine only has 7 worlds, most of which are fairly similar to one another. The gameplay, while enjoyable, seems to feel more "realistic" (for a Mario game, anyway) than the fun, cartoonish elements of 64 and Galaxy. The floating platforms, insane environments, and illogical situations of other Mario games just aren't here. Also, very few classic Mario enemies are in the game (Goombas, Koopa Troopas, Thwomps, etc. are gone) and those that are (such as Piranha Plants and Cheep-Cheeps) are barely noticed; actually, there's not many enemies at all in the game.
Overall, the game is pretty good and, by all means, worth playing, but it feels somewhat rushed and inferior to other Mario titles.