I used to a skeptic, to the point where I thought it was completely ridiculous. Then I realized that I was over-using inline styles anyway since I was always afraid of messing with old CSS. I was adding layers of overwrites just not to accidentally change something unrelated.

Now I can't imagine myself ever writing regular CSS again. I just love how I can look at the HTML and instantly "visualize" how the component will look just by seeing the classes. No more mysterious cascades where it's hard to pinpoint where the styling is coming from. It makes iteration so fast and responsive design has never been easier.