Ladies and gentlemen... step right up!
Anyone who's played Roller Coaster Tycoon, or any tycoon game for that matter, will find this familiar. You make your own theme park, you name it, hire staff, build stuff, etc. You start off by picking one of four guide characters, each with a different personality. They'll help you out a lot more than the guy in the original. The original game was very difficult, but thankfully they've not only toned down the difficulty, but included harder ones for more experienced players. The game's interface benefits greatly from the DS touchscreen. You'll find everything from stocks, loans, visitor feedback, charts, and much more to help you out. It's a VERY deep game, but at the same time accessible to casual players. You can play it nice and make sure customer satisfaction is the most important thing, or you can be a bastard and sell more drinks by upping the salt in the fries so it makes people thirstier. There's GREAT replay value, and there's dozens of attractions and utilities to build. It's gets pretty complex, balancing things like profits and employee needs, but it never gets too overwhelming.
The graphics are charming and expressive, and also pretty funny. It's a pretty old game, but now that it's scaled down to the DS screen they look crisp and less choppy. The sound is forgettable, but doesn't detract from the experience.
An exclusive DS feature is local wireless. You can visit eachother's parks and exchange messages. If you can find a friend that has it, it's kind of a neat feature.
In closing, you'll definitely see why this is on GS's Greatest Games list. The gameplay is superbly designed, and is a great fit on Nintendo's handheld. Don't be fooled by the quiet release or the crappy box art, because this is one of the most deep and well-crafted games on the DS.