My problem with the Bootstrap grid is the strange choice of breakpoints, especially on the smaller screensize end. It's way too complicated to alter the given breakpoints or add custom ones with Bootstrap. I'd rather use something like Susy for grids. Not so much more effort to set up, but much less limitations. Are you really all ok with Bootstrap's breakpoints?
IMO the smallest (fluid) size kicks in too early, but it's not hard to customise it. You can download a customised build[1], or override the LESS[2] or SASS[3] variables and build it yourself or incorporate it into your asset pipeline.
Personally, I use the SASS build, include only the most basic parts (scaffolding, type, grid, ...), customise the hell out of the variables, and you'd never know it was bootstrap. For reference, http://www.recruiteasy.com.au is a bootstrap-based site, using standard breakpoints.
I used the SASS version, too. But I found that in the end all the customization took me more time, than I usually need to set up the site without Bootstrap. For me Bootstrap is now more a helpful guideline than a practical codebase.
It is super easy to overwrite breakpoints in the variables, or just make your own. Especially if you use Less to define your grids and not HTML, you can just reference whatever you want, like:
