As someone who wrote HTML and CSS from scratch and then put a little time/effort into learning Bootstrap, I can do so much more in the same amount of time it took me to do my old layouts, and it looks so much better and is actually responsive.
All that to say, there is value in "becoming an expert" in a framework.
> All that to say, there is value in "becoming an expert" in a framework.
You can spend time learning Bootstrap's eccentricities, or you can learn something useful like OOCSS. One will make you really productive at making websites that are instantly recognizable as Bootstrap-based, the other will make you productive implementing any kind of design.
Time spent fighting Bootstrap is wasted time from this perspective. Time spent fighting your own designs is time spent learning valuable lessons about front-end coding. You may gain a marginal speed improvement with Bootstrap, especially (and perhaps only, I find Bootstrap slows me down) if you're new at front-end, but you're paying a steep price for it down the line, if you ever plan on building a site that doesn't work with the framework.
All that to say, there is value in "becoming an expert" in a framework.