Bravo! I recently learned that Greg Knese has chosen a free-to-download book, Real analysis for graduate students, 2nd ed. / Richard F. Bass, as text for his fall class Math 5051: Measure Theory and Functional Analysis I. Although the format and license of this particular book are not compatible with customization, which is one of the advantages of using open educational resources, it should still be a good experience for students.
Even if student choose to purchase a paper copy at $9.95, this will be a huge savings from alternatives, such as, Real analysis : modern techniques and their applications, 2nd ed / Gerald B. Folland, list price $178.47 (although there are less expensive rental options).
Students who use the pdf as their textbook may want to exploit this opportunity to explore free pdf annotation and pdf conversion software which will enrich their online experience in the future as well, since more and more stuff is online-only. The ones I have a little experience with are Calibre and Mendeley, but there are many others. I've read good things about PDF XChange Viewer and Preview (on Macs). ProfHacker often has posts about such tools.
If you have used or plan to use an open textbook, please do let me know so I can try to get the text added to our catalog, etc. Open Textbook Library and OER Commons are good places to locate options. More info. Thanks!