I've always hated reading documents formatted for paper on-screen. Until we get 200 dpi LCD screens, there aren't enough pixels to make small print-ready fonts legible without a lot of zooming, at which point navigation becomes a pain. And printing out a paper just to read it once, or maybe mark it up with comments that I then have to transcribe back into the online version, always makes me feel slightly dirty.
Thankfully, Word 2003's reading layout means whole forests can sleep better at night. It's "just" another way of viewing your document, but they've done all the little things right – fonts are resized to be legible, margins are widened to make lines easier to scan, and there's no scrolling between screens. Apparently reading layout also turns on ClearType by default. The end result is that I now prefer to read Word documents online rather than printing them out. And I get annoyed with PDF documents, which are stuck with a page size and format that's great for paper but sucks for screens (except a rotated Tablet PC screen).
My only annoyance with reading layout showed up in specification review meetings. When everyone else was talking about page 11 out of 16 in their printed copies, I never knew which screen (out of, say, 30) that mapped to in reading layout. So I started this post thinking that I was going to end it with a Word feature request, for an additional page counter in reading layout that shows the corresponding page numbers from print layout. That turns out not to be necessary: a little aimless experimentation just showed that Word's go-to-page-number function uses the print-layout page numbering scheme, even in reading layout. So now I can hit <Ctrl-G> 11 <return>, and another twig is saved…
Update: Apparently Bill Gates uses it too - and makes twice as many comments when reading documents using reading mode.