I prefer Weave Sync (http://mozillalabs.com/weave/), as it syncs settings, open tabs and browser history as well, and since, like Xmarks, it encrypts everything and lets you use your own server. I hope that Weave Sync one day makes it into Firefox trunk, as it's an extremely useful extension that goes well beyond what most bookmark/password sync extensions do. Sadly, it is limited to Firefox and Fennec right now.