It's fairly good. Good alternative to other Readability products that have been taken down or discontinued. Other than that the freeware Balabolka is a good alternative to read web pages and most other documents.

This is the only reader I could fine after the built in reader mode in Firefox take over the job. But it does not force itself in certain webpages as other previously widely used reader mode extensions can not. This is the only extension I find working for every website and web page.