Works well for *.doc files, haven't tried other file formats

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)

Generally, it works as excepted. Some files cannot open.