It's very hard to FILL Kirk's shoes, of course.
Chris did a very good job playing Kirk. At first I did not accept him, but then quite a few times during the movie when the camera angle was just right and he had certain expressions I was finally convinced he does look like a young Shatner at some moments.
You cannot expect the entire cast to look like everyone younger as well as Spock did.
GREAT REBOOT!!! I WANT MORE!!!