First, I want to say that what I consider to be the needed features of a notebook may not be yours. But for my uses, Quiver seems promising, but alas, cannot provide a much needed feature for me: portability.
Quiver uses a JSON-based backend. This is good since it can be placed under version control. But, it leaves zero room for editing the notes without Quiver. I keep my notebooks in a git repo. 99% of the time, I edit them on my mac. But there are times when I have to edit either on a linux machine or on my mac but over SSH. It is really not possible to do this.
Instead using a purly markdown based notebook has been much easier. I can still include code and images to my hearts content. I can also still use MathJax for LaTeX. But I am not limited to using this app. Plus, in the (unlikely) event that something breaks, either in the app or the OS, I am not locked in. I can move my notebook anywhere at any time.
Now, if that limitation doesnt bother you (though I think it *should*), Quiver seems like a great app. Cleanly built with good design.