I'm on the road the vast majority of the time, so having my entire library accessible via my iPad, iPhone, or Kindle Cloud Reader is immensely useful. That said, I hate the ebook reader format. It's fine for reading cover-to-cover, but it's a pain to search through. Footnotes, tables, graphics, etc. can get messed up too. My favorite option, which publishers like No Starch Press are great about, is buying a physical copy of the book and getting the PDF, mobi, and epub formats for free. I'd almost always opt for a PDF on the iPad if given the option.
The day you stop learning is the day you start becoming obsolete.