It's really hard for me to come to a definite opinion of Brendan Routh because, honestly, the movie didn't use him very much - I'm struggling to find a scene at least equivalent to the Clark Kent/Lois "Not unless you can fly!" moment. The critics were right when they said that one problem with the film was that Superman just didn't get much dialogue.
At the same time, I agree with others that said that Brendan Routh's awkward, quiet and invisible Clark Kent did much better than the stuttering nebbish Chris Reeves envoked.
The reason I personally always liked Christopher Reeves was because he had so much chemistry with the rest of the cast. Obviously there was Margot Kidder and Christopher Reeves (who no doubt had so much chemistry on set because OFF-set, they didn't get along) but also there was Chris Reeves being the straight man to Gene Hackman's comedy bits. Only one line in the film hints at the previous Hackman/Reeves chemistry:
"I see an old man's sick joke."Even the flying was wrecked for me because they replaced George's leap and hurricane woosh effect in favor of floating around like Peter Pan.
Really? I thought Christopher Reeves flying was one of the better parts of his performance. The guy knew something about how airplanes work (he was a glider pilot), so when he wanted to slow down, he brought his arms out wide to create drag. His back and cape moved a certain way when he wanted to go up, and so forth. It reminded me of the weightlessness that Gil Kane gave his Green Lantern when he floated like a soap bubble.