Here are some points others haven't mentioned:
(a) Uses canvas by default. Faster than any other library I've used.
(b) Extremely flexible. Want to write your own widget on top of the graphs (eg., a customized tooltip). Possible!
(c) Provides a lot of metadata. Want to get the position of a point in the rendered chart, to use in your code? Entirely possible!
(d) Works really well when importing into legacy web apps. They even provide an option to get a customized build on their site.
(e) Very good at handling streamed data. The animation is very smooth between data changes.
I've been using it for almost 7 years now for various production and personal projects, and it's still my go-to library. Their docs have come a long way since then.
They do have long standing bugs that get annoying, e.g, dealing with 0 when using log scales, provided there are workarounds for it. I haven't noticed any blocking bugs for most common usecases.