This is amazing. Seriously: I had a look at nearly every game engine/GUI solution in Rust and this is by far the most thought out, ergonomic and complete thing I found.
I've tried using Amethyst a few times but never finished anything. Upon each attempt I ended up moving to another language/framework due to the frustration of horrible compile times. Also, while there are lots of comprehensive docs, but the docs were written for a variety of versions so there was a lot of friction to get something simple started. It has been several months since my last attempt, so perhaps Amethyst has improved since then. I do love the promises Amethyst makes and for the same reason I'm very interested in Bevy.
I gave Amethyst a serious try about a year ago. I even joined the Gitter intending to contribute, as I needed some more dynamic rendering features, and read the foundational research on the wiki that seemed to inspire the project.
I found the application of the research -- the execution -- too early in development for my needs. I have the feeling it will grow into something more flexible with time, but at least back then, I wasn't able to render to a texture being applied to a mesh. Not being able to get around the deeply integrated ECS was hard, as well.
I tried Amethyst but found the API had quite rough edges in practise, and there was too much boilerplate for my taste. I certainly had the feeling they did certain things in certain ways not because it was the best solution, but because they didn't find a better way.
This was a while ago so it might not be fair as things probably improved since then and one should definitely give it a try as well.
That being said the whole bevy way of doing things looks very well considered indeed — maybe both projects can profit from each others existence?
Color me stoked!