My experience with Xmarks was not a great one. I used it when it was first called Foxmarks and enjoyed it. Some time after the name change, I noticed that one of the updates had turned on by default some extra overlays on the google search results page touting their rankings.
The blog post suggests that this was a bonus feature, or a value-add, but that's not how I saw it. A previously well-behaved bookmarks sync plugin all of a sudden decided to impose on my preferred search engine. That's a spammy low-class intrusion into my browsing experience.
I turned off the feature, but every time I updated the plugin thereafter it was turned back on by default. I jumped ship as soon as Chrome presented a viable alternative.