"This means when a change occurs on one Google Chrome client, a part of the infrastructure effectively sends a tiny XMPP message, like a chat message, to other actively connected clients telling them to sync."

Source: http://www.chromium.org/developers/design-documents/sync

It's problematic if you have multiple clients and had previously used something like Xmarks. My meticulously curated folders and bookmarks get messed up across devices pretty fast, even after deleting sync data and starting fresh from a clean backup of bookmarks.