There was a time that I really respected Arlen Specter. As the Commish notes, he was regarded as a principled independent centrist in the GOP.
But over the last year, I lost respect for Specter. His move to the Democrats was not out of conscience; it was because he was being challenged from the right and knew he was going to lose. Once he joined the Dems, he became the perfect Democrat, therby losing his independence. He thought moving over the Democratic column would be a safe harbor from any challengers, only to be beaten by an insurgent from his left who called him out on his cynical move.
I don't share the belief that some Republicans have of Specter as a traitor. Most conservatives never liked Specter, so the charge is somewhat baseless. That said, I do fault him for his rank opportunism. Democrats in Pennsylvania have rejected him for the same reason.