MX vs. ATV Untamed is an accomplishment on the DS, simply due to its spot-on recreation of the series in portable form. It nails the signature gameplay and offers a decent handful of courses and options. On the other hand, it lacks the wealth of extra goodies and the massive selection of courses and riders you'll find in Mario Kart, or even the console versions of Untamed.
You're getting a slimmed down, fine-tuned version of the game with perfected essentials and not a whole lot else. More casual players might be turned off by this, but hardcore fans can expect the essence of what they love about the series, in a size they can carry in their pockets.