It's still a classic, but not THE classic.
Pokémon Diamond and Pearl are very much classics, following the same formula I know and very much love. I was an avid player of my Pokémon Red, loved my Gold, and greatly enjoyed my Ruby. However, something seems to be missing from the fourth generation of Pokémon - the game feels less epic, and more formulaic. The Pokémon seem less inspired, the Elite 4 and Gym Leaders easier, often having only 5 Pokémon instead of the full 6. The legendaries are... ugly, and not exciting. If you're looking for a great Pokémon game on a Nintendo DS, look for Heart Gold and Soul Silver.