I'm a paying Dropbox user (for personal stuff) and I use Google Drive and Box.com at work. Dropbox is far and away the easiest to use. Case in point, my very non-technical grandmother once called me because she was worried there was a virus on her computer, only to find out that she accidentally changed the homepage on her browser. But when it comes to using Dropbox, it's easy for her to navigate and look at her pictures.