I used Clef to secure a personal blog that I run. Really nice, slick UI, and fun to show to people because it's so different. I had to uninstall it, though, when I had broken my iPhone and switched temporarily to an old Windows Phone I had laying around. I never reinstalled it, because an authentication method that relies on having a specific piece of expensive technology isn't all that attractive to me. Maybe it's great for someone who will always have an iPhone or Android phone, but in the last two years I've had five phones, and three of them were platforms that Clef doesn't support. This "iOS or Android" nonsense might work for games, but for any application I need to rely on for my workflow, it has to work anywhere. FirefoxOS might be the next hot thing, and if people switch to it, they're going to switch away from Clef.

At least with Google Authenticator other people can write compatible applications for other platforms.