An essential app Rated 4 out of 5 stars
XMarks is a very good sync app I've been using for years. On occasion (as others have reported) it can get confused by something you change somewhere and end up changing your bookmarks. Personally I have never lost my Bookmarks but it is always wise to export them once a week and to back them up.
Another very useful feature of XMarks is a starred review box where you are allowed to rate a site from 1 (worst) to 5 (excellent) and even write a quick review which you can even publish on Facebook immediately, thus making the need for extra like/dislike apps (such as WOT, which have been reported by many users to slow down browsing) nonexistent.
I give it 4/5 due to the hiccups it occasionally has in terms of bookmark 'confusion'. It happens rarely but some people seem to have lost their bookmarks because of this.
So long as you export and backup your bookmarks once a week, though, I think Xmarks will soon prove to be an essential app, esp. if you often use more than one device to browse from.