Pocket Monsters SPECIAL (aka Pokémon Adventures) closely follows the story of the original role-playing games while also adding its own subplots. The playable characters of all the games star in the manga within several story arcs that correspond to each generational release of Pokémon RPGs.
Story author Hidenori Kusaka did a marvelous job creating distinct personalities for each of the main characters and developing each story arc in a way that makes sense in the timeline of the RPGs. Even the creator of Pokémon, Satoshi Tajiri, himself said that the manga most closely conveys the world of Pokémon he was aiming for. Just like in the RPGs, Pokémon live and die in the world of the manga as well as have organizations that take extreme actions to get what they want. It's much more intense and gritty than the anime ever could be.
Personally, I prefer Mato's art style in the first 9 volumes over Satoshi Yamamoto's from Volume 10 onwards. I feel Mato's art style is more appropriate for the battle-oriented world of Pocket Monsters SPECIAL than Satoshi's, which made the manga have more of a shojo feel in the later volumes.
My only issue with the North American English translation of Pocket Monsters SPECIAL is primarily with the Red-Green-Blue arc, where Green and Blue's names are switched around due to the fact that Pokémon Green was never released in the U.S. Pokémon Green's lack of a stateside release should not have been a deciding factor in whether or not to keep the manga consistent in translation since it's not like they're going to completely edit out the third character in that arc to correspond with U.S. game releases. A character name Green was going to be in the manga regardless and I would rather have it belong to the character it was originally given to (the boy, not the girl).
Overall, I highly recommend this manga to anyone who has ever loved and enjoyed the Pokémon RPGs and franchise.