I recently signed up to take an online class at my community college, and had the opportunity to buy an electronic book for the course. The e-textbook could either be read on my computer or an iPad/iPod Touch/iPhone using an application named CourseSmart Bookshelf. This e-book seems to be more of an interactive PDF, since you can zoom in for a closer look on any page. On the Mac, I can also highlight text, and use the "Edit > Speech > Start Speaking" menu to have the words read to me.
On the iPad version, there is no search feature, but users have the ability to highlight and mark notes. The iPad/iPhone version ONLY works with an Internet connection (it logs you in whenever you start the app), whereas the Mac version allows you to read "offline" ... however, the iPad and Mac applications do not seem to sync to one another (similar to a Kindle that will return users to whatever place they were last reading in the book, without regards to the device being used.)
The overall cost was less than the printed version of the textbook, however, note that users do not get to keep the electronic text -- it's actually a 360-day "lease." I paid $41.50 to lease the book (which lists for $103.44 for the printed version). In addition, I had to pay a $50 fee to access the online lecture content at McGraw Hill (not part of the ebook). If I bought a new book with the online content as a "bundle" at the college bookstore it was $167.65, (or $125.75 used). I also considered that a trade in value on used books, is only between $10 and $26.
A recent update to the CourseSmart "app" (towards the end of the semester) introduced a new feature: the ability to connect with fellow classmates "within" the context of the book. This option was not promoted by my instructor, and I had none of my classmates' contact information in order to connect to fellow readers, so I'm not sure how this feature works.