As good as it ever was...
That being said, Heart Gold takes Platinum's game engine and slightly modifies it to augment the touchscreen functionality, this is a nice touch no pun intended) and it adds a seamlessness to the whole experience by putting the start menu options on the touchscreen, instead of in the start menu.
The game comes with a small peripheral, the pokewalker, which in my opinion didn't add much to the game for me, however, it can be used to catch rare pokemon and level them up to an extent right on the walker. The unit resembles the poket pikachu pedometer games that were released in the late 90's early 2000's. Overall though the game stays very faithful to the originals, and no matter which copy you buy you still get all the Kanto and Johto legendaries (in other words no ds crystal version for that). If you have HeartGold you can get Lugia without trading, the same holds true for SoulSilver.
The long and short of it is this, if you liked them before you'll still like them now. Also, you can find an item to make all the music the way it was on the GBC, which to me rocked out loud. Why not revisit a foreign land again?