Yesterday I spent 10 minutes throwing together an example WebExtension to show how a few APIs worked. It's not a complicated or sophisticated one by any means, it just shows a pop-up and links to trigger some tab APIs.
But when writing an add-on like that, you might notice a few things:
Thanks to about:debugging you can load the add-on straight from the directory and run it. There is no xpi, zip or compile step. It just works.
Those factors alone make developing that add-on so much easier than before. We are getting there.