I currently use https://materializecss.com/ over several products. It's been great, but I'm stuck on the 0.100.x line because the upgrade path was too difficult. It's also hasn't been updated since the first 1.0 release some time ago. Not complaining since I got what I paid for (it was free) but it's left me pretty anxious to find a replacement.
Salesforce's Lightning components are good but challenging to understand how to work with outside the walled garden. Zurb's Foundation UI is interesting but seems to want me to use whole page templates rather than components.
This looks like exactly what I've been dreaming of. I've put my money where my mouth is as I can't wait to have more components built out to a stage where I can start using it for real.