My overall impression was that it was definitely and surprisingly usable for non-technical people like my mom, grandma, etc. who don't use their computer for anything sophisticated but also don't have the time/energy/wherewithal to debug and configure things.
I can't speak highly enough about the Mint Cinnamon experience, and I recommend that everyone involved in the "Desktop Linux" world try it (at least in a VM) so they can get a sense of what "good defaults" actually look and feel like.
This was also my experience.
The only customisation i've done is:
1. Moving the panel, depending on what my feelings about proper panel placement are at the time.
2. Removing all the default shortcuts, because they collide with IntelliJ and/or are useless, and defining a few of my own
3. Setting up custom compose key sequences
Removing the shortcuts was done in the UI, but i really wish i could do it in a config file instead, because it's a pain to spend ten minutes clicking around. I had to hit the command line to set up compose keys, although i think this is an X problem, not a Cinnamon problem.
https://projects.linuxmint.com/cinnamon/
I've been running Cinnamon on a few machines for years, and it mostly works fine, which for a Linux desktop is high praise. The file picker has thumbnails (although it only has a list view, so they're ant-size).
I haven't used MATE. My understanding is that MATE started life as Gnome 2, whereas Cinnamon started life as Gnome 3 reskinned to look like Gnome 2. Both have grown considerably from their starting points.