Basically Heart Gold/Soul Silver made under the GBA engine, graphics from the DS Generation and the sound font transitioned very well. Kanto is not available to revisit, but the Johto region was made to be more complete. Wide variety of Pokemon fron Gen 1-4, more options for Dark types, while Fairy types are also included. Still some bugs in the game, which unfortunately may not be resolved anytime soon.
But this worth playing, especially on GBA hardware