Works well, but the usability is bad. You can't control whether it will intercept the request or not. I'd like to have some “kill switch” to turn the add-on off temporarily, or perhaps some additional items in the context menu:
“Open this link with Google Docs viewer” (whatever the URL is) /
“Open this link as an ordinary Web site” (if the URL ends with *.doc but e.g. it is in the local intranet or requires authentication)

It would also be nice if it were possible (e.g. by using about:config if it is not trivial to implement) to tweak the service URL's or add custom ones.

I, for myself, once have hacked the same functionality using a configurable sharing extension: you right-click on a link and there you find “Share on Facebook”, “Send to Pocket”, “Send with GMail”, --and-- “Open in Google Docs viewer”. (It's just an idea to the Author)