I have used Xmarks for a couple of years. I track the same tech stuff at work as I do for my blog and personal research, so synchronizing is useful. With Xmarks about to disappear (not so likely now), I quickly installed Firefox Sync. It seemed to work, at first, but then one of my machines got into a state where all its bookmarks were gone, and it couldn’t get them back from the Firefox sync server.
I eventually fixed it, by configuring a different Firefox instance to push its bookmarks everywhere, then I was able to get the info onto the losing machine.
Xmarks is not perfect either. On my machines with 512MB RAM and one CPU, Xmarks ties down the machine completely while it syncs. Can’t do nuthin!