Setting aside the ethical arguments for and against reading comic scans from the Internet, what would be a good reader for the common file types?

Almost all comic scan formats come in either CBR or CBZ format. These are really just RARed or ZIPped image files. There are a variety of viewer programs but the one I've come to use is Comix.

Comix is about as intuitive as a program can get. It automatically unpacks the archive and all you have to do is flip through the pages. There are some rudimentary keystrokes to display the pages in full-screen mode, actual size, fit-width, and as double page spreads.

To install Comix,

sudo apt-get install comix

To read the CBR files, you also do need to install a RAR unarchiver. To do this,

sudo apt-get install unrar

After that, you're good to go.