While not the worst game, it isn't as good as the rest.
Now, Ruby and Sapphire isn't much different than the other games, so I will not go into the details, so I will comment on stuff from this game only.
DOUBLE BATTLES:
Cool, but I prefer single. I still like playing them though. The only problem is that all the opponents have a grand total of 2 Pokemon between them, while I have 6.
ABILITIES:
Makes some Pokemon even better, like Koffing, while some new Pokemon suffer from their ability, such as Slaking. It still is a plus.
AVAILABLE POKEMON:
It is very hard to build a team due to the extreme lack of good Fire Types. Why do they keep making regions with very few Fire types?
DIFFICULTY:
It seems harder to level up and build a team. I kept losing the second Rival battle since she had a Fire/Water/Grass team before I can get decent Fire or Grass type. And suddenly, the levels of the AI trainers jump from the low-to-mid 30s to the low 40s!
REGION:
I like Hoenn, and it is much larger than Kanto or Johto before it, and I think it is bigger than Sinnoh, but I could be wrong.
The main problem is that I don't think the Gym layout and availible Pokemon is good.
OVERALL:
While it (along with Sapphire) is the worst of the Pokemon series, it is a good game. If you need something after beating HeartGold/SoulSilver to hold you over till Black & White, this is a good choice.
I give Pokemon Ruby (and Sapphire):
9/10
B+