I thought Grievous was pretty fun for what he was, and he had a lot of character for someone with so little screentime, the problem was that there wasn't much room to develop him because he only was in 1/3 of a single film. Now, that's okay, but the damn trilogy is so crowded that every single villain is like that. On his own, Grievous is a cool character IMO, I think it's more the context that makes people dislike him. There were too many useless, undeveloped, CG characters with spinny acrobatics and funny voices and no room for a more developed, realistic, adult characterization. Dooku had so much potential but he ended up on a development arc that went right into a brick wall, even if his death scene was really cool.