I don't use a framework, pretty much evaluated bootstrap for one project a year or so back and the time it would have taken to make any custom changes negated any benefits we would have got from the savings of a framework.

My opinion hasn't really changed there. We've used Bootstrap (and others) for things like internal dashboards, tools etc and they're great for that.

I use my own "starter kit" that has evolved over time to just be a kick-off point with some useful mixins, a grid and a few other bits and pieces along with grunt tasks etc and then build on top of that.

I'd like to make more use of Flex and saw a great talk from Stephanie Rewis at Industry Conf on using it extensively on the Salesforce Lightning Design System with great results but if I remember correctly they still use a normal grid right now.

I think BEM is becoming a well established convention and it makes reasonable sense. I keep meaning to update our own coding guidelines to fully embrace the markup but haven't got round to it yet but it's nice to have something solid and well thought out to work to after so many years of everyone doing their own thing.