It's Pokèmon, it's fun, and it's not getting stale. For these reasons, Pokèmon Diamond is a great game.
The graphics are reasonably good. On one hand you have the colorful presentation, stunning 2-and-a-half-D over world and the great attack animations, but on the other hand, some sprites are recycled from the GBA games, which really does not matter much, but still. The soundtrack and over world music is okay, but some Pokèmon cries are also recycled from the GBA.
The new features in Diamond impress, with Pal Park enabling you to transfer your Pokèmon over from GBA games to Diamond, the Pokèradar telling you where shiny Pokèmon are (among other things), touch screen controls, the addition of 42 new TM's (Items that teach Pokèmon moves), as well as the addition of the Sinnoh Region, and, the biggest addition of all, wi-fi battling and trading. Other features returning from the previous games include the day-night system, Pokèmon Contests (bad as ever), Pokèmon swarms and the basic story-line. (Won't go into detail about it).
The quest to collect all 493 Pokèmon has never been more fun, but still has its flaws, like the fact that the story is bland and short, and the aforementioned recycled graphics and sounds, but Pokèmon Diamond is a superb game anyway, so who cares.