I’ve recently started on the UI+GUI in my game, and can I just say: Impressive. The way nodes can be nested and anchored makes it so easy to, without any pain, create flexible user interfaces. It’s the best UI system I’ve used to date. Respect to the Defold team to make something this magical, and putting a smile on my face.
That was all.
GUI is such a good system you can make games with it and mostly ignore GOs.
That’s how most of the first big successful games made with Defold were made… 
Of course GOs / collections are still good and very useful. If GUIs allowed async load / unload for nodes, you could set material constants more easily per node, and you could assign scripts to individual nodes, I’d probably use GUI even more…
For example I can brag with my Matcher game created whole in GUI for my game purposes. It was so convenient and it effortlessly solved my problem with positioning the board where the hero (that can move) was. Aand it can be arranged for different screen size and orientation with ease 
I replaced factoring blocks with just clonning one reference block and changing its flipbook animation 
What other UI systems you have used before?
Absolutely not to nitpick or start an argument, I’m just genuinely interested
Not that many, to be fair. I used to work with Flash and Shockwave, and more recently I’ve used Gideros, which doesn’t have a visual system for UI. I’ve also tried a couple of web layout tools, like Webflow and Squarespace. All of those could have done with a bit of Defold magic!
I suppose Flash is the most relevant one, since it used to be (and maybe still is?) used as a UI tool for games on many platforms, including Unity, Unreal and CryEngine using Scaleform.
The thing that works best for me in Defold is how easy it is to nest nodes and to make them react to screen size changes. This was also possible in Flash, but not without a great deal of fiddling and code.
I just discovered nested GUI templates, and let me say again: 
The most unexpected thing here is that it all works as it should and you don’t have to look for some assets each time you find another non-working feature. Still my is for atlases. Incredibly great tool even without trimming. And LUA tables of course. =)