I actually liked Brandon Routh take as Superman. I think he played it well. I don't know why folks have a problem with his acting in Superman Returns.
It's not so much his acting, although I don't remember enough about his performance to have much of an opinion on it, which may be telling in itself. For me, he just doesn't look remotely like Superman. Some people seem to think he bears a striking resemblance to Reeve, but to me they look nothing alike. Routh just doesn't have a face or a voice that fits my image of Superman. I could see him as Sherlock Holmes, or maybe Spock (aside from the voice), but I was never able to see him as a credible Superman.
I had a similar problem with Dean Cain. He worked as the version of Clark Kent they were going for on that show, but he didn't work as well as Superman. Partly because his face and voice were both a little too distinctive, so it was blindingly obvious that Clark and Superman were the same guy, even more so than usual.