In my brief testing with it installed, it was most useful with older content that had been given a chance to be passed around the Web. Newer items, especially from niche blogs, had no related discussion.

For heavy Google Reader users, this is an extension that’s definitely worth installing. It doesn’t actually do any of its magic until you hit the “comments” button that’s added to the Google Reader interface when installed. This means it’s not going to slow down the initial load of your feed, or interfere with things like Gears.

From a CNET article about GReactions