it would seem to me that it would take much more craft to make a character we don't know believable, rather than go, "Ahah, that Spacecy sure can do a great Christopher Walken."  

Eric, I really don't follow you here.  I think either situation would require tremendous acting skill to do well,  the difference being with the  "character we don't know" being more open to the actor's interpretation of that character.  If anything, trying to portray the character we do know would be open to more scrutiny, simply because we expect certain nuances in the portrayal of that character.  And it's more than just a voice imitation.  It's the subtleties, body movments, facial  mannerisms etc... Sorry, can't agree with you on this.

And...I have heard that Jamie Foxx's Ray portrayal is so striking, that if you have seen films of actual Ray Charles, you will be convinced that it is Ray on the screen.   Contrast that perhaps with Val Kilmer's less than convincing (though interesting) part of Jim Morrison in the movie the Doors.