There are only a few franchises in the gaming industry that you can name drop and everyone, instantly, knows exactly what you mean. It's like Tetris, or it's a Doom clone, or, in many cases for handheld games, it's a pale shadow of Super Mario Bros. These practically need no introduction, and New Super Mario Bros. continues that proud tradition.
Other than porting over some of the plumber's moves from Super Mario 64 (butt stomp, wall jump and triple jump), this plays just like the classics you remember. There are eight worlds, divided into separate stages that contain power-ups, hidden exits and a bevy of colorful enemies to stomp on. At the end of each level, there's even a flagpole to scale, just like the original NES game that inspired an entire generation of gamers and developers.
If it's relatively unchanged from something we all played 20 years ago, why bother? Well, if Nintendo can get one thing right, it's a side-scrolling Mario adventure. New SMB still has that magical sense of exploration and broad range of environments to keep the turtle-stomping and block-busting interesting. These games routinely challenge you to think outside the level, and consider multiple ways to reach the end - that means hours of fun.
Then again, it's mere hours that it'll take to put Bowser back in his place and rescue Peach once again. If you're good, you can completely wipe this out in one day, secret worlds and all. The fact that the game practically hands you extra lives on a silver platter only makes it easier - not a complaint, but tougher bosses would have been welcomed.