Bootstrap is a beast. However, this is my approach to it though. I have never been into frameworks but I gave bootstrap a shot and it is a beast and you won't use all of it. I did however spend hours playing with portions of it and taking apart what was applicable to me and removing unnecessary IE * hacks. In the process I learned a lot. The end result was that I was able to create a "template", not so much a framework for my projects.
Less is really useful in this though. So I know that pretty much every project I have will use form elements, navigation, search (since I do a lot of Wordpress - so that context worked for me).
I was able to pare it down to a template that had basic resets and styles for forms, nav and search etc... combine it with Less, I could tweak either rounded corners or not, borders, colours etc for all of these elements in just 3 lines or so and I did not have to reinvent the wheel for each project.
This approach might not work for everyone but I would say, at the very least, studying Bootstrap will do you a lot of good and maybe spark some ideas on how you can do it better.
My only caution is that you do not use it as is. You can tell a Bootstrap site by looking at it and that is a result of someone just using Bootstrap as opposed to adapting it.