Pokémon Ranger brings unique gameplay elements to the table, but still recycles some story elements.
In Ranger, you have an interest in helping to preserve pokémon versus capturing and training them like in the "normal" games. You use a Capture Styler to encircle the pokémon and pacify them to become something akin to a friend that you can use to assist you in the field (such as using a quilava to burn down a log) or use them to help you capture other pokémon.
The story has you solving various citizen's problems while pacifying the pokémon involved. Eventually, a "Go-Rock Squad" (read as Team Rocket clone) start causing trouble.
Gameplay wise everything is a nice change of pace. You're sent on various missions where you must pacify pokémon. Pacifying them involves using your stylus on the screen to draw circles around them x number of times. You can use pokémon to help in capturing other pokémon. It sounds simple, but for me it was a nice idea to introduce into the pokémon realm.
Sound is disappointing. There is new music, but all of the pokémon sound effects are recycled from the other games.
Also included are several bonus missions which eventually leads to the discovery of the Manaphy Egg which from there can be brought to any Diamond or Pearl game and raised there.
In the end, Pokémon Ranger feels like a side quest to Diamond and Pearl souly for the purpose of the Egg. It is entertaining while it lasts, but there's not much left afterwards.