You're probably pretty likely to get a response from Maynard or the people on myspace. If they respond personally, it might take a while as they probably get quite a few letters.
From Bob Iger, you'll probably get a response from an assistant. But I'd be willing to bet you'll get a response.
As for Johnny Depp and Keira, you'll most likely get a response, but it will probably be from their publicists. I interned and then worked for a PR firm in Hollywood who represented everyone from Charlton Heston to Aerosmith to Sam Kinison. When I interned we read the letters that came in. We were told if there was something really really good or disturbing to let their publicist know and they would determine whether or not to pass the letter on. We had stacks of autographed pictures by the celebrities, most autographed by them and some that were signed by the interns. A lot of celebs like to hear how much fan mail they received, but most just don't have the time to even read much of it, much less respond to it.
The best time to get a personal response is when they're up and coming, before they've "made it".