It's a good mario platformer, but not quite the revolution we've been expecting.
The gameplay is classic as well, with some tweaks that make mario even more powerful than before, which may or may not be annoying to you. As far as level design goes, it's really good and a lot of effort has been made to make levels different from each other, so there is always surprises and new obstacles for the player to overcome. The only exception is those middle of the way towers and mini bowser battles, which get annoying after a while.
Sound is also great, the player character's now get voice overs similar to those in SMB64 and other newer games, and music is good too.
Now for the weak side, the game is a bit short if you simply breeze through and don't care about secrets and stuff. Probably you won't be spending much more than 1 hour per world that way, which equals about 6 hours to finish the game (6 worlds are needed to beat the game). If you thoroughly explore the game multiply this by 2 or 3, depending on how good you are and so on.
For another positive thing, this game also comes with 18 touch screen minigames, which can be played at any time you want, unlike SMB64 where you had to unlock them before hand. Although the total number of games is less than in SMB64, the good ones are in (i like card games the most).
So in the end, the game is good fun, the graphics is great, the levels well designed but a little short. Regardless, this is one of those games every DS owner should have in their collection. It's still quite a bit away from perfection however. NSMB 2 perhaps?
8.5/10