My app of choice has been the kindle on iphone and android. Most people do not realize you can get many of the benefits of the kindle on another device simply by using the app.
I also use Aldiko for android for ebooks I have purchased.
The best part about reading a book electronically is that you can easily bookmark and come back to things later. Searching across a book on these apps is almost nonexistent but they all claim to have more robust search coming soon.
The other benefit is synchronizing purchases across devices.
Many people say the drawback to eBooks is the inability to share them with others. I have not found this to be a problem, and have found it actually stimulates book sales.
Recently a friend of mine polled a bunch of us via email saying he was going on a long trip and to please recommend books to BUY on his kindle. He did not ask to borrow the expensive hardcovers and paperbacks but rather purchase them himself. This seems like a common practice that I hope continues.