I prefer the feel of a book, but they're efficient.
Paperless - Good for the environment. But, the charging of the device isn't green. Luckily, Kindles last a long time if you use them properly.
Multiple device access - I can access the book I bought for my sister on her Kindle, or on my phone since she registered it to my Student Prime account.
Portability - One device the size of a book can hold hundreds, maybe even thousands. The Kindle Fire also has all those cool apps and stuff.
Cheaper - Downloading a book can be free, or ~1/3 of the cost. Got a $15 book for $4.69. Prime accounts let you access the lender library and get free stuff. Also, if you authorize an account, you can convert a .pdf into a Kindle format. We did it for a few books I have as .pdf.
Etc etc etc.