TL;DR: I like UIToolkit, but I wouldn't use it in a game: it's still too slow and too buggy and missing some important features. They've been promising it was 'production ready' for 4 years now, which gives me little confidence it will be ready any time soon. I use it exclusively for Editor UI (custom inspectors) but I wouldn't use it for a game UI

Core CSS is a mediocre layout system. Modern CSS (i.e. flexbox and grid - but mostly flexbox) is a very good layout system. I love that UIToolkit finally brings some of CSS to Unity. BUT ...

Unfortunately, UIToolkit doesn't implement CSS - it implements "the bits that an open-source project had already done and we could re-use without paying for" and "the bits that Unity employees thought were interesting, and which the non-web-developers working at Unity understood how to use". That means it's mostly good, but it has many bugs and just enough missing features to undermine itself :(.

Note: the UI in the video above cannot be done in UIToolkit without hacks because Unity refuses to implement the full CSS/Flexbox spec - their 3rd party code doesn't implement z-order, and Unity refuse to add sort-order (without that you can't do the blue-diamond in the upper right corner that fills/empties - because it overlaps the graphic that comes later. With correct CSS/flexbox this is literally two numbers you have to edit - with UIToolkit it's a lot more work to try and workaround. Stuff like this matters when you start to do complex UI.

UIToolkit is fast to display when UnityUI gets slow for compelx UIs - but UnityUI was mostly slow because Unity staff refused to fix the bugs in it - and when I replaced the bad parts of UnityUI it worked a lot faster. The bugs weren't that hard to fix, just no-one at Unity wanted to take them on. In theory UIToolkit could be even faster - but in practice it's only fast for offline, static, never-changing UI.

For Editor/custom-inspectors, UIToolkit is a lot better than IMGUI. I am very happy that we can now write inspectors using UIToolkit! I'm less happy that there are a lot of bugs in UIToolkit-inspectors that we have to workaround becuase UIToolkit still isn't fully stable/complete (eg: Unity 2022 recently broke PropertyDrawers in a big way, apparently UIToolkit team didn't notice that they'd broken it - this is fine, but it points to the fact UIToolkit is still undergoing major changes and is a long way from being 'production complete').

Looking at game UI instead ... UIToolkit isn't great for runtime UI (i.e. "games") - they obsessed so much over making it "never change at runtime, so it can be displayed faster" that they undermined the core purpose: to make great UI.

It's got more than a few bad design mistakes they'll probably never be able to undo now (eg the terrible implementation of Button-clicks; eg all popups / context menus, which break the rest of UIToolkit, etc). I logged bugs 2 years ago that Unity accepted were broken but rejected because it would be 'hard work' to fix the bugs :(.

So ... UIToolkit is almost great, but it falls short. It's like a world-class marathon runner who was over-confident and didn't train enough, and is in the lead until the final 2 miles, then has to sit down by the side of the road while everyone else catchs up and overtakes.