It would be nice if we could have a comparison with the already existent options.
There was a google extension (now dead) for the same thing, and then appeared FoxMarks, now called XMarks.
I've been using XMarks since the google extension died, and I am a very happy user.
I would like to know why would someone like me have a reason to change. It seems security is better, but not really sure (XMarks only encrypts the passwords list on the server, not the bookmarks list).
From what I see, XMarks has the advantage to have plugins for multiple browsers: Firefox, IE, Safari and Chrome (beta). You can also have different profiles, like one for home, other for work and another for mobile devices.