Souter is a lightweight, and as an outsider lacked the strength of character or intellect to resist the pressure to "evolve" as a justice.
None of these traits apply to Roberts. He has solid character, a brilliant mind, is firmly grounded in his religion, legal philosophy, and family, and has lived in Washington for many years.
I am unconcerned that Roberts is a Souter or that he will "evolve" while on the bench.
Particularly since he has served in Republican administrations. It was noted last night that executive branch service is one of the best ways to ensure a judge does not evolve since they have set views that don't change.