Yes Semantic UI has come a long way and changed since I first used them 1-2 years ago. I have just tried it on a new project their annoying build process and lack of official SASS support the style and out of box components are really great.
I created an app for a customer in it, and while it looks shiny, it would have been more cost-effective to just have used bootstrap.
It seems like a good start for prototyping and admin interfaces, but for anything else it just adds too much styling and classes that need to be overridden.
The name seems poorly choosen, since you're constantly adding classes that reference styling decisions making it everything but semantic (ui very wide, ui very thin, ui three column doubling grid, …).
I have had some experience with semantic UI components recently and I enjoyed using it.