I’ve used it, both svelte and sapper.
My opinions:

The good:
It’s reactivity works well, you get used to the special syntax bits very quickly, so don’t be afraid of it.

Overall it’s quite fast, both for those synthetic benchmarks(look at how I render a million dom nodes with my library instead of using canvas!) and for real use cases.

It comes with a state management solution(stores), animations and transitions out of the box.

Sapper makes sense, and is a fast and quite batteries included solution for both SPA and SSR. You can get an decent SPA with SSR website up and running in a couple minutes. Handling cookies can be a bit confusing at first, though.

It’s relatively easy to integrate with other js libraries, I wrote some bindings for prosemirror while learning it and it worked quite fine.

The bad:
It’s output doesn’t work in legacy edge nor older browsers(try to see any NYT article featuring Rich Harris visualisations on an older browser, it just doesn’t load).

There’s no way to access child nodes. If you want to wrap individual childs(say, receive a list of components and wrap each of them in some other component, like you would do with special list items), you can’t.
For example, I created a ListStream component that imitates discourse’s cloaking behaviour(unmount out of screen dom nodes), and I had to pass the list items as a prop for the ListStream to wrap them, which is a horrible workaround in my opinion.
There was a discussion about that in the svelte’s discord when this article was published and there was no solution. Responses from contributors were too defensive from my pov(part of that was valid: the medium article wasn’t particularly good, but it had valid criticism on svelte’s lack of reflection) and ranged from “yes you can” or “why would you do that”, but no examples of how is it possible was given, and it was proven to be impossible.

That discussion and Rich’s responses in his Why I don’t use web components article, where people addressed his criticisms and he replied with snarky comments or refused to continue the discussion left me with a bitter taste. I would like to see more transparency and discussions about svelte’s weak points too, so they can be addressed properly.

Typescript support was bad a couple months ago, though it is supposed to improve sometime in the future.

Overall it’s very good, and it’s worth trying out.

I’m not currently using it though, since I want to render as much html as possible with Elixir Phoenix and custom elements are more suitable for that usecase.

We’re at an opposite end of the spectrum working with Ember.js. Kinda maximalists

Sadly never worked with it – just heard a lot of good feedback from JS devs.

It’s kind of like Rust for JS, meaning that it compiles to “native” JS without you having to carry the framework with you. Which I quite like.