I don't use a particular framework like Bootstrap or Foundation but have my own project folder customized to suit my own needs and work style. Everytime i start a new project i just make a copy of that folder and start working from there (I have it set up so that I can invoke a create project command from Sublime and it makes a copy where I want).
I prefer doing it this way because I can have everything tailored to my workflow, as opposed to having to learn and adapt to a particular framework.
But generally I tend to not use one for a more complex project. Feel there's way too much in there that I just don't need, so why overload my pages with unnecessary code. Plus if something goes wrong you have to really dig deep into the system to find out what's causing it and how to fix it, this sometimes messes something else up..
18 months ago, I tried using Bootstrap for a project, just to see how good it was.
I now use it in everything. Literally every web site I'm responsible for uses Bootstrap and FontAwesome.
I'm still on Boostrap 2 just now. Moving to Bootstrap 3 and FontAwesome 4 requires some manual changes to your HTML, especially with the changes to its grid structure. I'm putting off the horrible task of replacing loads of .span6 with .col-md-6. Maybe this summer.
It's usually dictated by the project I'm working on.
I built my own SASS-based setup for a recent project that was built in the Sitecore CMS. When I work in Adobe CQ/Experience Manager, I usually pull in Bootstrap or Foundation, and on a series of flat marketing pages I'm building, I'm using Foundation.
It's good to know how to build a solid fluid grid system yourself, but having an off-the-shelf solution is sometimes good for speed.