It's very obvious that smart people are using BS to build cool site like khanacademy, it's well designed, and it's one of the best CSS framework you can find. But...
But for a CSS beginner like me, it's a bit too magical, I wasted a lot of time debugging, I wished I could conquer the complexity, so I tried for 6 months, and now I give up with regret.
Bootstrap(or maybe I should say CSS frameworks) is slowing down my development for the past 6 month or so, if you are a front-end newbie, be warned!