Meh.
It works fine most of the time, but sometimes it can create duplicate contacts. On top of this, if you follow celebrities or even non-people like news stations on Twitter, it will create contacts for them that I cant seem to remove, since every time it re-syncs it pulls it back down again.
The native sync in Mountain Lion and iOS 5/6 works the same way and has the same flaws, so I dont see the point of using this over the built-in apple solution.
last.arabesque about Cobook, v1.0