Darth Venal said:

The irony there is that Ewan McGregor said he was trying not to really impersonate Guinness too much "as it interferes with the acting process".

Being an actor myself, I can attest to the truthfulness of this statement. If you feel like you're doing an impersonation of someone, it prevents you from getting to the "truth" of your character, which is something I suspect Ewan, and many other PT performers, would have been able to do were it not for Lucas:

Taken from the Wookiepedia article on Fan criticism of George Lucas:

Harrison Ford, who played Han Solo in the original films, said in an interview "I think George [Lucas] likes people. I think George is a kind, warm-hearted person, but he can be a little… impatient with the nature of acting, that need to work until you find something."

Bingowings said:

McGregor argued that it would be pointless to try and do an impression of Sir Alec because people don't sound the same when they are young, while that is true there is still a signature in the voice that never changes and if he was trying to attempt that it failed.

That is so true. I just saw Surrogates last night, and there is a scene in which Bruce Willis is meeting in an office with an actor playing a youthful looking surrogate being controlled by James Cromwell's character. I was very impressed by how well the actor managed to mimic Cromwell's speech patterns and intonations. Even though the general tone of his voice was not the same, I was convinced that this guy could have been James Cromwell in his mid 20s.