How to use this?

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

Why.....

Doesn't work, extension failing to capture any requests, browser just opens save file dialogue.

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)

Didn't work. just shows codes!

very good

Works just fine, very useful.

This extension is unparalleled. Please upgrade it.

So far so good. Very handy!

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

says no preview available