Was good, not so much anymore. Rated 2 out of 5 stars
I've been using Xmarks since it was Foxmarks and for years had no problem. It's installed on a total of nine different browsers on three different computers (two of which dual boot different operating systems). It has always been reliable--up until three or four months ago. Now it hoses my bookmarks at least once a week. Usually it destroys the folder structure--making multiple copies of folders most of which are empty and none where they are supposed to be. Fortunately it does allow you to restore old backups from weeks or months ago so it's fixable but you still end up losing some bookmarks.
Bottom line: Find something else to sync your bookmarks. Firefox sync is a pain but it has to work better than this. Chrome also has sync built-in and it's easier to use than Firefox sync (no "pairing" different computers with codes and what not--it just uses your Google account).