Better than the original, with dozens of hours of battling to be done.
Starting off, the story is little different than it was before. The most interesting pieces of story are your rival's progression from being a hearless trainer at the beginning to having a sort of lightened resolve toward the end of your journey, and the story of Team Rocket and their plans after the disappearance of Boss Giovanni. Particularly, this game takes place 3 years after Red/Blue, and so Gold/Silver acts as sort of a direct sequel of sorts rather than just a new installment. Team Rocket hasn't given up their plans and wants to use the radio to serve their purpose of taking over the world, and you happen to end up the trainer to stumble upon them and stop them. The last time you take on Team Rocket is sort of anti-climactic. While there's quite a bit of intrigue there, there's absolutely no challenge, as Giovanni would wipe the floor with the admins that were left behind. However they do serve a nice purpose of adding an entertaining story, so that's a plus.
Graphics are a nice step up from R/B. With full color support on the Game Boy Color, the sprite-work and the world map have great design to them for a Game Boy game. However the world map looks a little sub-par even by Game Boy game standards. As expansive as the Johto and Kanto regions are put together, there's a lot of eye candy left to be desired. Still, that doesn't stop the places from looking pretty nice, and it doesn't stop them whatsoever from being memorable. If you have been to Cerulean City before you'll never forget the Gym's placement, smackdab in the near center of town (though a bit off to the right). The attack effects are also quite nice to the eye, and the battle engine is where more of the astheticly pleasing stuff will come in.
Sound was a great improvement over R/B, which were great to begin with. However, Pokemon cries and some attack sound effects are screechy and just plain annoying. I have been known to mute the game at times due to this. However, the battle music is no better in any Pokemon game then here. Between the Johto region's battle themes and the Kanto region's remixed themes from R/B, you're going to enjoy those battles even more than you did in the first set of Pokemon games. For those new to the series, the music here is, again, no better. It's just plain catchy.
Finally, there's actually things to do after the game is said and done. You can trade Pokemon and battle them, and that actually will lead to a specific trainer in the game adopting the team of Pokemon the person you battled or traded with. Think of it: that level 89 Charizard and 91 Blastoise your pal had in his game up for battling. Every day too. Nice concept, right? In addition, for each time you defeat the Elite Four, you'll be able to battle Pokemon Trainer Red, who's team is supposed to be like Red from the manga (or a little bit like Ash from the anime). On top of that, the clock and pseudo-calander (it can't tell months but can tell days of the week) add special events only available at a certain time, and also add the element of night to the game, which is later adopted by Pokemon Diamond/Pearl. A lot of the great features you see in future games started in Gold/Silver.
Gold/Silver doesn't have everything, and it will eventually grow old, but when there's 251 Pokemon to collect, catchy music to hear, tons of battling to do and 16 Gyms to conquer, you'll have your hands full from the time you pick the game up to the time you put it down. And it'll be addictive fun the whole way through.