I have been using ReadKit for a couple of years now and I love it. Lots of features and access to various services like Pocket, Readability, etc. Performance is great. No problems at all.
Someone mentioned CPU usage. It's in the background right now and it's using 0% CPU. I'm currently running it on OS X 10.11.3
When I had some questions about the app, the developer was very responsive.
I highly recommend it.