We need more professors to use this text. Of course there are drawbacks to an online textbook. You can't use it as a door stop, you can't use it as a booster seat. If a rabid dog comes charging at you, you can't bop it over the head with it.
But then again, the online version is free, it's over 1500 pages but can fit in your pocket (assuming you have a flash drive), and the author can update it at a moment's notice.