It's hard to rate this one, it was a big part of my childhood. But it doesn't age well.
But it all boils down to this, you had to own the original first. It was a milker. It was a novelty back then, and a load of fun if you had friends over. But playing the game with the bare minimum (single player with no red/blue) can make the experience a lot more dull. The only fun in that would to improve your pokemon skills and learn all the tricks in the game, thus I use the term, Pokemon encyclopedia. And ESPECIALLY by todays standards, this game is lack luster, because we have 4x the pokemon now (I don't even know I don't follow it). And sure, there were some mini games thrown in the mix, but again, not so fun playing by yourself.
This is a game that's really hard to emulate, you have to be growing up in a time when pokemon was the thing, and everyone had their pokemon red/blue, and could take their pokemon and duke it out in the stadium. Thats what it was meant for. But somehow by adding other gadgets to it, Pokemon Stadium turned into its own game. And when that happened, the game took a turn for the worse.
Thats why GS gave this a low score. You simply can't understand it as an adult, or a reviewer. But as a kid, Pokemon Stadium was the stuff. Pokemon Stadium is a perfect example of a game that doesn't age well.