I bet it'd be this bad in reality, too
Simple: you hook up some device to your controller that has a microphone so you can speak. You hold a button, speak into it, and then it shoots a bubble at PIkachu. If it hits, that means he heard you and he'll doing something. Pick that up, come back, follow me, etc. The game is played in a first person perspective where you control... yourself. You can pick up objects, throw them around, etc.
All you'll really do is take Pikachu for a day out running around collecting items and... fishing? These activities are kind of amusing and are actually fun until you've done them for the 300th time. Plain as that. It's interesting and cool how Pikachu actually recognizes your commands, but it's a cross between something cool and something completely stupid. What I really wanted would be some battle simulator at least where you shout out what attacks he could do, but that doesn't cut it.
You can pretty much forget about the graphics now that we're two generations farther. And all you'll be hearing is mediocre music and yourself cursing into the microphone (which apparently isn't recognized).
I can't imagine you getting this game anytime soon, so I guess if your kid or something only has an N64 for entertainment when he should have a Wii insetad, get them this game. They'll be terribly disappointed, but that's exactly what I felt when I played this game seven years ago so it shouldn't matter.