I decided to write Pong using bevy to learn it a bit. Eventually I plan to rewrite https://github.com/mkhan45/ssshmup to learn the engine a bit further so I can contribute.
The way I've structured some of the stuff was kind of experimental and didn't work out quite as well as I'd hoped so I probably wouldn't use this as a good example.
My biggest problem with Bevy is that its documentation is lacking and it doesn't have geometry primitives built in, but bevy_prototype_lyon does the job and the documentation should improve eventually. Bevy ECS is very ergonomic and apparently much faster than specs, so I'm tempted to try using it with ggez.