In theory ePub is a wonderful format. Ari Armstrong, in a recent post to his blog, talks about the benefits of ePub:
I like the idea of the ePub format, developed by Adobe. It is open, so anybody can use it. At least theoretically, any author or publisher can create an ePub, and anybody can create a reader for the format; several readers now exist. ePub already reads on a variety of devices -- including my iPod Touch -- and I hear several more compatible readers are entering the market. Unlike an HTML ebook, ePub organizes many files, including text and images, into a single package. Unlike pdf ebooks, ePub reflows text to fit your screen and reading preferences.
Armstrong then points this out, "The problem is that it is a royal bitch to create a complicated ePub book." You can export to ePub from InDesign CS5 (IDCS5.) But, what you get is really bare bones. Exporting from IDCS5 is just the first step. Then, you need to go into the document and start editing it. There is an open source WYSIWIG ePub editor named Sigil. It is great, but WYSWIG tools for creating ePubs have a long way to go to be main stream.