As mentioned above you need to understand html and css to a good standard before you can get the most out of frameworks like bootstrap.
There is a steep learning curve when using something like bootstrap but that's mainly learning how the grid works and what all the built in features can accomplish for you. Once you understand that then it's still up to your html and css skills in how you manage all this together. Sometimes you may want to come out of the grid and create your own structures and sometimes you can just go with the defaults.
I would almost never use bootstrap on small projects as there is just too much overhead for a small site and in reality it takes probably takes less than 10 minutes to knock up any sort of initial layout anyway. For large sites with many developers bootstrap can be a boon as it encourages a consistency between developers and makes it much easier to manage and indeed quicker in the long run.
Some of the benefits of bootstrap are it in built features such as dropdowns, modals, hide and show effects, standard controls, tooltips and many others that are available with a quick cut and paste and can save a lot of time re-inventing the wheel.
To be honest I've never been keen on grids because they never seems to suit the design in hand unless they've been specifically designed to suit the grid and most times I can do it quicker from scratch anyway. However, I have recently built a couple of hundred pages using bootstrap and I didn't really run into many issues but I did avoid using the grid classes for the complicated pages.
In the end its just another tool and some developers will say avoid bootstrap like the plague and other love it but most would admit that you should learn html and css to a reasonable standard before you start to rely on something like bootstrap.