Diamond adds plenty of fresh material, and online trading, making this the definitive pokemon experience.

User Rating: 10 | Pocket Monsters Diamond DS
To me, Diamond (And Pearl, same game sans a few pokemon) represent Pokemon at its best : A fun story (nothing too major, just fun), new challenges, and of course, the classic addicting "Gotta catch 'em all" formula with 493 pokemon.
I'll repeat that. 493. 493 little, and big, critters to capture and train. That's every pokemon to date, plus the 100+ they added for Diamond and Pearl. I've played for 146 hours, and I still haven't gotten them all yet (I'm close though).
Enough of that, review time.

Graphics are 2D and 3D graphics, making your character a fathead. But it makes the world delightfully fresh, and feels like it should, being on the DS.
Pokemon battles are abound with new attack animations, and new attacks. Otherwise, same old pokemon. But that's not a bad thing necessarily.

Gameplay and controls are the same as before. Gameplay is the classic 'build-a-team and battle it against other trainers' style of play. Leveling up is the same, but with new evolution styles and evolutions, you're in for a surprise, assuming you haven't read online or in a guide of how something evolves.
Controls are standard DS affair, with a few touch screen options, but nothing major. In that sense it feel a little lacking, but not horribly so.

Story is the same formula since Red/Bue on the original Gameboy : Boy (Or Girl) meets a professor, gets a pokemon, and journeys through the world, fighting gym leaders and the generic bad guys, eventually becoming the champion. While this isn't anything new, it's still plenty of fun. Each gym leader has a strategy, and honestly, this is the most challenging pokemon I've played in a long time.

Returning features from Ruby/Sapphire/Emerald are the Battle tower, where you earn points to buy items and stuff, and a ton of legendary pokemon.

And the coolest feature EVER in pokemon history? Online trading. Seriously, this feature rules. The way it works is this : Get a wi-fi connection, and either go to the Global Trade Center or whatever, and put pokemon up for grabs, or find one that you really want. Or do the better way, and go to www.serebii.net, and go to the wi-fi chat room. In there, you actively trade, like a market, for pokemon, and when you find someone willing to trade, you trade your Friend Codes (Nintendo's safety feature for online use), and trade away. Now instead of taking forever to find a person in your neighborhood who has a pikachu, you can trade online!

So, with all the cool features explained and whatnot, should I critique this game now? Ok, here it goes.
Nothing is wrong with this game, and everything is right about it. Good challenge ratio, a massive collection of pokemon, and online trading make this worth the $35 for it. If you have a DS, and don't have pokemon yet, saying "It's for kids" or "Pokemon is dumb", go buy this, and enjoy yourself for the next 146 hours.
This game is a triumph of Nintendo game genius and simple RPG fun. It's addicting to the point of needing to go to rehab afterwards. This game belongs in every DS owners collection, regardless of age.