Emerald is the third installment in the Hoenn region. If you haven't played Ruby or Sapphire, this game is for you.
As always, you go out on a Pokemon adventure. You have a rival you fight, you collect Gym Badges and thwart a diabolical scheme by Team Magma and Team Aqua.
The story around these two teams is very interesting, as both are trying to bring two very powerful Pokemon out of hibernation. You don't choose sides, in fact you tend to get stuck in the middle of their plans all the time. Once these two powerful Pokemon are awake and fighting each other, you have no choice but to go find another legendary Pokemon that is the only way to end this battle that will ultimately destroy the planet if they are not stopped.
I found this story to be well done. I have no comparison to the R/S story but this alone made this game very interesting. It brought back memories of Team Rocket and how you were always thwarting them. I like this a lot more than choosing a side, as R/S had you do.
One of the disappointing things I noticed in this game was the lack of night and day, a great addition to the Pokemon games in the second gen. It felt like a step back at first and was difficult to get used to. This is not a very big issue though.
Leveling up your Pokemon can be difficult at times. Sometimes you have train for long periods before you can move on and fight a Gym Leader or just prepare for the next route. The biggest disparity in this is the Pokemon League. There is no excuse for needing to train for an hour or more just to get your Pokemon at a respectable level. This has been in every game so far and I wish it was corrected so it wasn't so out-of-balance.
There are EVs and IVs, things I have not become very knowledgeable with but are new to the training scene. It's a feature that requires you learn about it and train for a long time to get what you want. Natures are also part of this as they affect EVs and IVs. Certain natures make it easier to train certain stats and others make it harder. Again, another new feature that requires you to take the time to learn so you can benefit from them.
Breeding remains in the third gen and is important if you want to try and get the right nature or just the right initial stats for your Pokemon. Once again, you have to breed to get pre-evolution forms as you cannot catch them in the game.
The graphics are awesome. You actually feel like you're in the region as you play because of the vibrant colors and the new look some things have. The new attack animations are simply amazing. Pokemon animations are great as well.
Overall, Pokemon Emerald is a great game if you haven't played Ruby or Sapphire and would be a good purchase. Like all Pokemon games, it is highly addictive and you can't put it down for extended periods of time without thinking about it.