The Clone Wars had to end with some plot point or pivotal moment. Using Grevious to do that was fine. I mean, it makes perfect sense from where I stand for the droids to have some type of field general for combat against the Republic. I don't like the idea of Dooku doing that so, Grevious wasn't too farfetched at all.
Would it have been cool to introduce Grevious in Episode II? Sure, but he still works even if you hadn't seen the Clone Wars, which I didn't.
As far as Anakin's turn, it's all about going forward to gain the power to achieve his goal. With him, unintentionally helping with the death of Windu, he can't go back to the Jedi. So, he might as well submit to achieve what he truly desires; the power to save Padme.
The turn doesn't work without Episodes I and II. The threads for that scene are in the previous films. Now, execution is subjective but I think it's a very good scene. And I also love the scene because it's the scene that Anakin begins his plan to rid the galaxy of Palpatine(a plan he continues in the Originals-overthrow his master to become the master). Problem is, he never to into account the Darkside.