Installed Size column is really useful when we want to know which packages are eating our drive the most, so we can sort it and uninstall
packages we don't need in case we're short of drive space. I know hard drives are getting cheaper, but sometimes we in the third world don't have that luxury, especially after the Thai's flood.
Download Size column is useful especially for us who have slow internet connection.
I know not everybody wants these columns so it shouldn't be the default. That leads to another suggestion: add a way in the Options to enable/disable columns? Maybe "Columns" tab?