The chrome color-changing is obnoxious and plainly harms UX, and the design is weirdly wasteful of pixels for something that looks like it was intended to save space. It also continues the flattifying trend of making it impossible to distinguish individual interactive elements unless you're looking directly at them—gimme buttons, depth, and contrast, damnit.
And that's after switching it to the more-classic-like version that keeps the URL bar separate.
The tab groups are sort of cool, but they're bugged for me (opening a link in a new tab results in a blank tab, after briefly displaying the intended content after I switch to it, but only in named groups, not in the default unnamed tab group) and their relationship or lack of relationship with windows keeps throwing me off. Overall this feels like something the window manager should handle, in some fashion, though I admit existing features aren't quite enough to cover what it does. It also seems like the whole concept needs to be integrated with bookmarks. All in all, good direction, but something's still missing.
Oh, and I also forget to switch to them, so haven't used them at all since about the first week after initially setting a few useful ones up. I need a way to at least tell Safari to open tabs for certain sites in certain tab groups, regardless of where I start to load them. Otherwise I end up with the same 100+ tabs in the unnamed tab group as I always have.