Billy West, the voice actor behind Ren & Stimpy and several of the characters on Futurama has some things to say about celebrity voices:

[W]ith your little strip of vocal cords, you're going to create heavens and hells and universes and populations of people, which is the whole idea that a voice person has in their head. It's like, "Whatever it is, I'll be it." But the voice people can physically escape the sound of their own voice. We do multiple voices. We used to save producers' asses, because they'd hire you and say, "Well, we were going to get six people, but we can't afford it. Can you do this, this, and this?" And you'd do them, and they'd be perfectly happy, and they'd save a bundle of dough. Now, it's the exact opposite. The minute they mention a CGI film, they're already looking to see what Renée Zellweger is doing. They're already looking to see what Billy Crystal is doing. This doesn't make sense, to do what they do--spend zillions on visuals, and then have this totally fucking flat-lining voice track. You know, "Hey, I'm Will Smith, I'm a clam! I'm Will Smith, I'm a kangaroo!" All you bring to the performance is your own ego. They're just being themselves. Let's put it this way: Cameron Diaz is the highest paid voice actress in history: $20 million for Shrek. Why? Because she has a 9-foot mouth? That works somewhere else, but not on tape! [Laughs.] It's like what the hell is that all about?

Let me say this about Billy Crystal as a voice actor: he was kind of distracting in Howl's Moving Castle, because it really popped out at you that it was Billy Crystal doing the voice, which is not good when you're supposed to be paying attention to the fire demon that he's voicing.