On the positive side:
* very complete, with good form styling, and lots of widgets you will use often, which is especially important for larger apps,
* the default theme is mature and has good usability, without the crazy "oh, how flat and invisible our UI is!" look.
* the class naming plays well with React (I use ClojureScript and Rum) and looks good in your code,
On the negative side:
* the CSS is huge and there is little you can do to trim it down,
* the JavaScript code is not Google Closure-ready, so it's a drag compared to my ClojureScript codebase: large and unwieldy,
* there is a jQuery dependency, so I have to pull that in, too,
* the build system is… well, strange, let's put it that way. I'm used to typing "make" and getting things built, while this thing here insists on a) painting pretty pictures in the console window, b) crapping node_modules in a directory up from the one I'm building in, c) requires interactive feedback. I still haven't found a way to automatically build Semantic UI from a zip/tarball, and others seem to struggle with it, too.
Overall, I'm happy with the choice and it has been serving me well.