I recently read Free Culture (by Lawrence Lessig), switching between a paper copy and the PDF version on a monitor (which was free - hooray for Creative Commons licenses!). Reading on the monitor was great for when I wanted to read during lunch - no need to try to hold the book and a sandwich at the same time. I also used the PDF version when I was somewhere with a computer and wanted to read, but didn't have the paper copy with me.

The rest of the time I would read the paper copy. I also print out longer articles to read - paper seems less fatiguing to read from than a screen.

The font, font size, column width, and line spacing seem to make a big difference with how tiring it is to read on the screen. The Free Culture PDF was one of the the easiest documents I've come across to read. The default WordPress and Blogger themes use similar column widths and line spacings - this suggests that there might be an optimum way to display text for readability.

This is a bit of a leap, but if you're designing a site that has a lot of paragraph-form text, mimicking the typography of a typical paperback will probably do a lot for your site's readability. Does anyone else have specific text formats that they prefer for reading on the screen?