Cav-El deserves more credit. Most critics LIKED him, but I don't think they were generally impressed by him.
Cavill had to make Superman his own out of a script that frankly doesn't do Clark Kent (the motivating character behind that persona) justice.
Routh got the worst of it, though. "Minneapolis City Pages: ”Brandon Routh is no Christopher Reeve, who was the same age Routh is now (26) when he first slipped on the Spandex but somehow possessed more warmth, humor, and gravitas all at once.”"
(Who could have been?)
I don't think Routh is a great actor, but not even Cavill would have saved SR.