Not quite as great as SMB3, but still an excellent game!
The New Super Mario Bros. is neither challenging nor long in length. To beat the game you are only required to complete 6 worlds, most of which have about 8 levels including your boss fights. The game is also not very challenging. Most worlds will be beaten in two tries or less and you can take this entire game down in a day if you do not have anything to do. I will not complain though because I actually enjoy an easy Super Mario Bros. game every once in a while.
The price of this game is not really too friendly. (30 or 35 bucks brand new) and it may be wiser for you to spend 18 dollars on Super Mario Bros., Super Mario Bros. 3, and Super Mario World on the Virtual Console. The fact that the New Super Mario Bros. is a game that you can take on the go is one of the great things that this game has. Overall, the game is solid, but not revolutionary. If you can borrow from a friend or find at a cheap price, I would recommend it though.