Great tool! The Firefox add-on works like a charm, also Google Docs integration is perfect.
But the LibreOffice extension (paid version) is crap! Even when following all the guidelines, it won't work. Some functions do, sometimes - until they crash. But all in all this is a big disappointment, and completely useless.
Maybe it does work with MS Office, but that's not really relevant here.
So, online it's great, offline it sucks! Therefore only 4 stars.

I don't understand why so many people think this should be totally free and use this argument for skipping a star. Nothing is free in this world, and the free version is already excellent for most applications and far more comprehensive than free versions from other companies.
When you are using this professionally, the price is better than fair. When you're using this for fun, be reasonable and appreciate getting great value for nothing in return from your side!