New Super Mario Brothers is a solid title, with high production values, vintage gameplay, great minigames, and solid multiplayer. However, I found the title lacking the charm, diversity, and most importantly challenge, of earlier entries to the series. Level designs are top notch, but a reduction in power-up variety compared to previous titles, and enemy diversity populating levels make for a lot of well-built, but repetitive levels.
Multiplayer is a welcome addition, consisting of frantic rushes to collect stars and steal them from one's opponent. This alone makes the game worth purchasing, even if the single player is a step back from earlier Mario titles.
Overall, NSMB is a good game. The single player is solid on it's own, albeit a downgrade from previous incarnations of the series. The multiplayer is excellent, and manages to turn the game into a "must have" regardless of my disappointment in the single player experience.