Pokemon Diamond and Pearl are the deepest, longest and overall best games in the long, illustrious franchise.
The game follows a nearly identical formula to the previous Pokemon games. Your a young Pokemon trainer who aspires to become a Pokemon Master. When the game begins, you'll meet your Rival and Dr. Rowan. Dr. Rowan will of coarse let you choose 1 of 3 Pokemon to start your quest with, and of course, your rival will chose a Pokemon too. Anyone who's played a Pokemon game before should be familiar with how this works out. The game also features Team Galaxy, basically what Team Rocket was to Pokemon Red and Blue and of coarse, it'll eventually be up to you to foil their evil plans. Sound familiar? If you've played through a Pokemon game before it probably should.
As a matter of fact, Pokemon Diamond and Pearl feature the same core gameplay featured in previous Pokemon titles, so anyone who has played a Pokemon game will know exactly what to expect. There have, however been some fundamental changes to the games battle system, that may seem insignificant, but can actually make a big difference during some of the tougher battles during the game.
As with before, Pokemon battles are still turn based battles where the two Pokemon will attack each other until the other loses all of its HP. And of coarse, the Pokemon's strengths and weakness' will play a big part of how the battle turns out (for example, attacking a fire type Pokemon with a water type move will do 2x the attacks normal damage, but if you attack a water Pokemon with a fire attack, it will only do 0.5x its normal damage). But now Pokemon attacks have been split up into two different kinds of attacks - physical and special. Basically, as you level up your Pokemon, they gain stats (Pokemon stats include HP, Strength, Defence, Speed, Special Attack, Special Defence). You'll notice that most Pokemon will have a significant difference in 'Attack' and 'Special Attack' stats. This is where the two different kinds of attacks come into play. Before, suppose you used an fighting type move, the game would automatically consider that a physical attack (and therefore the Pokemon's 'attack' stat would influence the amount of damage done by the attack), but now that there are two types of attacks, some fighting type moves are classified as 'special' attacks (and in that case, the Pokemon's 'special attack' stat influences the amount of damage done). Additionally if a Pokemon uses an attack which is the same type as the Pokemon (e.g., a fire type Pokemon using a fire type move), the attack will get an additional boost of power.
These seemingly small changes can make a surprisingly big difference during some of the harder battles in the game (such as the Elite Four) and especially when fighting online against other people.
Speaking of battling online, for those who don't know, Pokemon Diamond and Pearl are the first Pokemon games to feature full Wi-Fi support, allowing you to trade and battle against other Pokemon players all around the world. The trading comes in handy especially if your planning on collecting all of the 493 Pokemon (actually, its nearly impossible to collect them all without a cheat device or Wi-Fi support). But trading online isn't quite as effective as you'd think. Not because of any technical flaw of the game, but you'll find that if you search for a Pokemon, most of the people that have it want a level 100 legendary Pokemon or something like that for it, which can really get annoying.
Pokemon Diamond and Pearl are easily two of the best looking games on the Nintendo DS. They don't push the DS hardware in any way, but the stylized graphics style combine with the amazingly vibrant and colourful colour palette in the game to create a really nice looking game. You'll also notice that buildings and other solid structures are polygonal which actually adds a lot to the games presentation. It helps make the game not look as flat as its GBA predecessors.
Similarly, the audio in the game is nicely varied and generally has a nice sound to it. You'll notice that different areas, towns and even some Pokemon battles have different sound track behind them. The audio is however, hampered by the sounds the Pokemon make, which sound ugly (if that makes sense) and really...not Pokemon like. Still, overall the sound compliments the graphic style pretty nicely.
If your expecting to collect all 493 Pokemon, complete all the side quests and unlock everything there is to unlock, plan to at least spend about 220 hours in this game. There is a startling amount of things to do in this game. Apart from capturing all the Pokemon, there are Pokemon contests for you to take part in, items to unlock, berries to find (which help out with the contests) ect.
So overall, if you've played a Pokemon game before and liked it, there’s pretty much no way for you to not enjoy Pokemon Diamond or Pokemon Pearl. The series core gameplay remains the same, but with enough changes to it to make it interesting enough for long time players to come back to. If you've never played a Pokemon game before, no matter how old you are, as long as you don't hate RPG games or something, you should play this game, there is a reason Pokemon games are the 2nd highest selling games of all time after all y'know.