This scene for me (you can skip to 1:20 or so) showed that it's at least feasible, but it definitely takes a lot more than glasses, which is where many of the non Christopher Reeve's portrayals fall flat. They always play Superman in both roles but it's just Superman with glasses.

Christopher Reeve on the other hand showed that Clark Kent is the perfect disguise when portrayed correctly and it's at least understandable that especially the people that know Clark would never suspect for a second that he's Superman because they've never seen anything but a quiet, weak and easily frightened man who is nothing like Superman at all! Superman is also way taller!

Watching him stand up straight and put some confidence on his face, voice and transform into Superman over the course of 10ish seconds was one thing, but watching him deflate back into Kent in under 2 seconds was another altogether! That shift was jarring for me and showed just how good the disguise was and how good of an actor Reeve was.