Meh... in a universe that has Jar-Jar, Boss Nass, the two headed pod race announcer and, if you count the Clone Wars, a german evil mad scientist, Grievous is almost cool.
Like pretty much everything else in the prequels, he doesn't make any sense. There's no way anyone could beat him in a fight when he's spinning four lightsabers super fast like some blender. Also he looks more advanced than Vader and much more agile.
And like a lot of other things in the prequels, he's only there so we can have yet another lightsaber fight, cause you know, having three fights in a movie is not enough, you need four or it gets boring.