He's clearly not playing it as straight as, say, Clancy Brown did in the animated series. If he were, threads like this would not exist.
The general consensus seems to be that he isn't playing it completely straight, which is what I was referring to. If he had evoked Brown instead of Hackman, I think most objective fans would be saying, "Wow, Spacey might end up being the definitive Luthor." (Personally, this is what I expected until I saw the trailers.)
I know everyone here has nothing but praise for the film, but if you go to neutral sites, the reaction is decidedly mixed.