I love Sunshine. I never owned the game (borrowed it twice, extensively), but I still want to get it sometime. Why? Because it feels like a huge playground. Super Mario 64 and SMB3 and all those games, they have a great layout and they're very well constructed and the objectives are challenging yet fun. Super Mario Sunshine has a reasonably good structure, but not much of one when comparing it to previous Mario games. Yet I have played tons of Sunshine and would still like to, just because it's fun to move in the game, because the controls are the most brilliant I've ever laid my fingers on.
I'm not going to say that it's better than Mario 64 or SMB3. I've played the other two more, but only because they've been out longer (and I had less games to play then, relying on my parents to purchase them for me). I just find Sunshine a very enjoyable experience, and like the setting, it feels like taking a vacation for a few hours whenever I play.
I think Super Mario Galaxy will blow it away, though. I think it will define, for me, the Mario game. That's kind of a highfalutin' thing to say, but I've seen the videos and it seems so fully to be what I've always wanted from a game, that it's hard to believe it could be otherwise.
I suppose I could be setting myself up for disappointment. I hope I'm just setting myself up for supreme satisfaction.