At Home Assistant we have used Lit Element to power our new user interface and it has been great. Small and fast components, easy to learn for new developers, intuitive API.

One of the coolest thing is directives, https://lit-html.polymer-project.org/guide/template-referenc... (you can also write your own). Directives are attached to a DOM element and can interact with the DOM as they see fit. Examples are awaiting an async iterator and writing results to the DOM as they come in, or have a directive that attaches an animation or ripple effect to an element on click.

For people interested in a UI built with it, the Home Assistant demo is at https://demo.home-assistant.io and source is at https://github.com/home-assistant/home-assistant-polymer/tre...