I chose Semantic UI and have been very pleased with it so far.
- Updates haven't been a concern for me because I haven't found it lacking or buggy.
- No annoying bugs yet... although I've only been working with Chrome and occasionally testing in IE 10 (I don't need to support anything below that).
- Browser support seems fine for me
- Yes, very easy to mix and match components. The authors seem to have done a really good job at making everything fit together nicely. Documentation is great too... lots of examples.
- I use react with it... but not out of need. React came first and I chose to add Semantic UI. Will write an article about this eventually.
My only complaint is that it requires jQuery. As a react user, I have no need for jQuery and would rather not include it.