Pros: Longevity, good replay
Cons: Addictive

Pokemon Yellow is classic RPG. This game doesn't have one main character, it has 151. As we all know by now, your quest in this game is to "catch 'em all." Along your travel to Pokemon League, you encounter other trainers with the same goal as you, become League Champion. Some levels in the game are actually challenging and it does take you some time to complete the entire game(excluding catching all 151). To catch all the Pokemon, will take you quite a while, because you need a friend with Versions Red and Blue to link with and trade. You can catch about 137 of the 151 in Version Yellow.
It's replay value is very good. As in many RPG's, your character(s) grow with the more experience they get. That is no different in Pokemon Yellow. This is what makes the replay so good. There are so many pokemon you can train to be more powerful, and with each one having it's own moves and become stronger, you just want to play it more and more! Which leads to a problem... The game is highly addictive. When I first got it, I was playing like a madman.
Another downfall of the game is that it may be too challenging for the little children, whom the game was designed. Tons(!) of kids under the age of 9 love Pokemon, and will have this game. It could possibly be too hard for them. I even got frustrated with it at rare times.
If you have a link and a person with any other Pokemon version, then you can battle it out with them! It is very fun, and you get into it. If you and a friend both have Version Yellow, then you can compete in a series of competitions to receive various awards(Cups). If you win a certain Cup, then in the begining screen, you can play Old-School Excite Bike with Pikachu!
This game is a blast. I recommend it to anyone.