It's relatively painless. The start I made (
https://github.com/ds125v/moodle/tree/bootstrap3) was at a point they were still moving pretty fast so there's been more stuff later. The diff looks a bit crazy, but that's mostly just dropping in the Bootstrap files on top of the old ones. The Moodle changes start from this point:
https://github.com/ds125v/moodle/compare/master...bootstrap3#L50L14
Basically none of the Bootstrap stuff has been added to Moodle core or renderers yet, so it's mostly a case of replacing the Bootstrap less files with the new versions, then doing some simple find and replace in the Moodle .less files and changing a few names in the layout files.
I would have aimed to use BS3 for 2.6, as it's a bit cleaner and more consistent, leaner and faster, and better for building on top of, but there still seems to be no real vision for adopting Bootstrap centrally so it's hard to tell what's going on. I intend to port it anyway as a separate theme and submit any of the changes I can as MDL issues, should be ready (as a third party theme) for 2.7 and probably work in 2.6 too.