For most usecases, bootstrap is fine. It delays the "designing" part so you can think about your product and actual features directly. And if you are tired of the "standard look and feel", you can buy nice themes later for a few bucks.

Projects that should not use bootstrap are the ones where every milisecond in performance truly matters, or when you have custom design requirements that would need major "overruling" of bootstrap. In this case, I recommend using a very lightweight CSS framework with a responsive grid instead, like http://purecss.io/grids/ .