It adds an reply with links below each of the comments under your photo (highlighted above). When you click on one of the links, the user name/buddy icon/both html code will be added into the Add your comment box, where you can show nicely who your replies are directed at. Experiment with preview and you will get the hang of formatting your replies in no time. (Should work on photo and discussion thread pages)
Thought the risk is minimal, where you can just tell Greasemonkey to uninstall or disable the script if it doesn't work, I just like to put in the disclaimer that you run this script at your own risk! Always backup your system first ;) Also it might not be happy with other GM scripts that manuplate the same areas on Flickr? It might be possible to make this work with Chrome and Safari browsers, but I do not test with those browsers.