The DS is perfect for Pokemon
Pokemon Diamond is one of the best games on the DS, whether you're a newcomer to the series or an experienced veteran. The battle system has been tweaked a bit, with new abilities, attacks, and the first real distinguishing between Physical and Special moves.
The game also allows you to transfer Pokemon from the GBA games like Emerald and LeafGreen, which really helps, because there's 493 Pokemon this time around.
You'll have plenty to play in Pokemon Diamond. Contests have gotten a massive overhaul, including new events. The new battle system makes real use of a Pokemon's stats, and strategies have changed. New monsters to collect and trade with friends add a whole new meaning to "Gotta Catch 'em All", and the competitive metagame is deep, whether you're training with advanced things like Effort Values, breeding, or anything.
The game's graphics are gorgeous, though nothing compared to the remake, Pokemon Platinum. Sound quality is also good, and a lot of the themes are catchy as the dickens.
The plot seems slightly re-hashed from the Hoenn region, and some areas are bland, but overall, this game's a near-perfect DS adventure.