As a point of clarification, this is an account based app that is giving a third party access to your data. If you are comfortable with it then it can be a good option. It is not, however, just a bookmark enhancement/add on.
Umm, hold on their slick. You are missing the point. It's putting your favorites on their server for the purpose of syncing them to any/all of your systems. You're not "giving them access" to anything.
There's no need for fear and paranoia. Read the product description.
And FYI, I have no affiliation with the company except as a user... of which I have been using it for going on 5 years now.
It's not about sync... it's about presentation, although syncing favorites is a benefit when you can use Xmarks across multiple browsers, not just IE.
The point is pinning this on the side pane and having a navigable favorites menu instead of the God-forsaken infinite scrolling in IE10 Metro is a great feature.