Ranked in these QuestionsQuestion Ranking
Pro The elements offer a huge amount of customization, far beyond a framework like bootstrap.
Pro Load only the components you need
Pro It's semantic
Uses semantic class names for its styling, making it easier to grasp and understand even for beginners looking to jump right in.
Pro Well documented
The documentation is easy to use, well written and has lots of examples each with their source codes.
Pro It'll have almost any UI component/element you may think of for your project
It includes tons of UI components that you may need for almost any type of project you may work on; And of course, you can always only pick what you need.
Pro Easy to use
Con Very large file size
Packages are much bigger when comparing to Twitter Bootstrap or Zurb Foundation. Semantic UI is really extremely large and it would be better to use specific modules and components, rather than the whole thing.
Contains a lot of UI inaccuracies, like wrong positioning, cannot mix classes, etc.
Con Small number of classes
Once you wanna do something that is not mentioned in the doc - prepare to spend an hour, then give up and implement a custom "workaround".