YES! They pimped the game majorly! It's my favourite pokemon game ever now.
The graphics are updated, the entire game system is according to 4th gen games I explained earlier. With the switch between physical and special moves and such, abilities and everything. They got some new evolutions for 2nd gen pokemon in that have to be get through special ways... like Piloswine only evolves into Mamoswine if he knows Ancientpower, which is a lv 0 move, so you need to get the MOve relearner to give him that move and then he evolves. There are new evolution stones as well, a shiny stone evolves Togetic into Togekiss(which kicks arse!) and Murkrow evolves with a Dusk Stone to Honchcrow, which is awesome as well.
You also have some new plot things, there's a Safari zone added now, you HAVE to battle Ho-Oh/Lugia before being able to go to the league, that story line is altered a bit with the Kimono Girls playing a bigger role. Won't spoil too much. Team Rocket has more depth to it too. After the first league playthrough they'll get MUCH higher levels and new evolutions and pokemon according to the National Dex, so there's an added challenge, the Gym Leaders will rematch you and you can find them outside the Gym on several special days and hours after you get 16 badges.
Everything is just pimped for the better, like Ocarina of Time 3D.
Oh and you also have something new called the POkethlon next to national park. You can easily get items like Heart Scales(for move relearner), Evolution stones(which were a pain in the ass in g/s/c, only bill's grandpa) and other evolution items like King's Rock(Politoed, Slowking) and Metal Coat(Scyther, Onix). So they majorly fixed the shortcomings of g/s/c/ without changing the game too much.
If you have a DS, I suggest you get a M3/M4 flashcart.
You can download the NDS roms on there and play them all on one micro SD card. I did buy HG and SS both, but for backup I also keep a Rom(making it legal). Though they're worth actually buying them.