The two good things about Pikachu in Yellow are (1) Tentacruel and Zubat are everywhere, so its not hard to grind him (2) he is the only Pokemon in-game to learn Thunderbolt by level-up. But other then that? He's decent in the beginning, falls behind later due to his low stats. I remember my Pikachu couldn't even overwhelm Loreli's Water Pokemon. I think the only E4 Pokemon i managed to beat with it were Golbat and Gyarados.