Those who rate it low and say it's bad haven't played it to its fullest extent.
I grew up with this game, and it landed a great impact on my life. Even after all these years, I still play it. This game is also the first pokemon game outside of Japan that shows pokemon in 3D.
There is no true storyline: just be the best there is, and then some. There are many game modes included in this, such as the stadium, or the GB Tower.
Stadium: The main event of the game. You get a choice of tournaments/cups, each one having it's own rules, such as one cup allowing pokemon that are smaller than a certain height. All of the cups have level restrictions, though. There are four difficulties for each, a harder difficulty being unlocked when the last difficulty has been beaten. Also, aside from pokemon you have trained yourself and transported to Stadium, you can choose among Rental Pokemon (Pokemon that can be chosen to battle, but cannot be kept or altered).
Gym Leader Castle: Based on the Gameboy games, more or less, this mode has you going through the Eight Gyms, each gym having 3 opponent trainers and the leader, and after the gyms, the Elite Four. You also are able to chose Rental Pokemon in this mode. There is a catch, though: all pokemon, rental and opponent, will be level 50, but should a pokemon from a Red, Blue, or Yellow game be used that is above Lv. 50, only the opponent's pokemon shall be the level of the trainer's highest leveled pokemon in their party. If there is a pokemon in the trainer's party that is lower then Lv. 50, it would gain no levels, nor would if affect the opponent's pokemon.
Gameboy Tower: A mode that allows you to play your Gamboy pokemon games on the T.V. a game is hooked up to the N64 via transport pack. Also, upon beating the Stadium mode a certain amount of times, you can adjust the speed the Gameboy game runs at.
Free Battle: As the name would imply, you can have battles here. You can change the rules to mimic cups, give the opponent any pokemon you want it to have, and even play against and with friends. This mode is unique as far as practice battles go. Since there was no such thing as double-battles back then, if you and a friend go as a team, you chose 3 pokemon to battle each (depending on if you want a max of 3 each), and take turns fighting your opponent/opponents.
Kids Club: One of the best features in this game that Colosseum, XD, and Battle Revolution missed. Mini-games! If you get a little tired of battling and want to take a break, you can chose one of 9 mini-games. You can play with up to 3 other people, computers, or a mix. There are 2 mode: one is just a free play: play any mini-game you want. The other mode is "Who's the best", where you have to win a certain amount of mini-games before the others do.
Hall of Fame: Just like in the gameboy games, pokemon that have beaten the game, be the pokemon a trained one or a rental, they will get a statue here.
Prof. Oak's Lab: This place allows you to check your PC, Trade, and check your pokedex if you have a game inserted. The PC system is much easier to use than in the gameboy games, and also allows you to store pokemon in the N64 game This is quite useful, as it allows you to move pokemon from one file to another without the need to trade. The trades are done quicker and easier than if you were to hook up two Gameboy systems and trade pokemon. The advanced pokedex not only allows you to view data for your pokemon and search for where they live in the pokemon world, but it also tells you how rare or commmon the pokemon is in the area, or floor of an area.
Overall, this game, along with its sequel, Pokemon Stadium 2, are the best console pokemon games. It's sad that Nintendo changed things around in later console Pokemon games. With all being said, this game deserves an 8.5. Even if you don't have a Gameboy pokemon game (like me several years ago) it's still great. It has a good balance of things to keep you busy by yourself, while at the same time, allowing you to have good fun with friends. This game is a must have for Pokemon fans and N64 owners.