Definitely what you think it is, a classic Mario game SM64 style.
The game is a typical Mario game with Peach being captured by Bowser, but Nintendo always seems to keep their Mario adventure franchises interesting with new twists.
To the average gamer, this may be just another Mario game to you, but it's still good at that, and if you're a Mario fan, you will like it pretty well.
The graphics look great on the 3DS, and playing in 3D is a lot of fun, but I usually keep it on 2D for my preference.
The Sound is a lot of fun, bringing back old-school tunes to new tunes, and fun sound effects.
Overall it is a very un game, that gets addictive, especially for 100% completion junkies like myself. The game has pretty good replay value, and will always be a fun game to go back to, when you haven't played it in a while.
The only negative I have to say, is that it could have been a little longer and sometimes the view can get frustrating since you can't change it, but it's still a fun game nonetheless.