Rated 4 out of 5 stars
It's a good add-on. It does what it's supposed to do, which is allow you to read .epub files. It is a bit lacking. It ain't full of features like others mentioned: text manipulation, color changes, font families, etc. could be added.
If you want something to get the job done and don't mind the dull look, then this is fer you. It won't crap up yer browser or aggravate you (it's less likely to do that).