Rated 1 out of 5 stars
Every once in a while Xmarks randomly decides that one of my passwords is "inappropriate for the profile" anymore. It then throws a fit and refuses to sync at all until I log in to their website, go over all my passwords and manually add to all profiles the passwords that Xmarks decided to suddenly exclude. Unfortunately it only works when their web site is available (it isn't at the time I write this review).
And to add insult to injury, even when it does work, it doesn't sync passwords because they want you to get lastpass for that.
So, your passwords are uploaded to their server, some synchronization operation does occur there -- since it often fails -- but nothing is saved back to your machine? Smooth.