I like it, as it's much more semantically oriented than Bootstrap. But why do the buttons still need the "BTN class?
tl;dr; It makes for cleaner grep results.
I like to use prefixes and base classes. It's faster for browsers to paint, leads to fewer rules overall, reduces selectors, and helps prevent unnecessary overrides.
codeguide.co also looks well worth checking out. At the risk of derailing the conversation further, what's the reason for leaving a trailing slash out of self-closing elements? I recognise it's optional, but including it gives you the option of XML-parsing your HTML; is there a non-aesthetic advantage?