(I just made a clean installation of joomla 3.3, with only the default bootstrap framework as well as the custom joomla bootstrap extended version enabled. Created just a clean template accommodating for modules and component calls with the default bootstrap styling applied.)

From the looks of it, the newest K2 is only partially responsive out of the box? Articles/images does not scale in Category view. Only in Single Item view and some views generated by K2 Tools and the like. Am I missing something here? It seems like k2.css line 422 is forcing a fixed width on the article image.

Don't want to complain, but If I have start by writing overrides for K2 just to make it bootstrap compliant, I think I will have to reevaluate my plan of using K2 on a site :(

You should look for a template which is compatible with K2 as well.
The article image's width is inherited from the category's settings.
All you need to do is to specify a max-width.

The same applies to the category, generic (used in user and tag listings) image sizes.

Finally I am pretty sure there are several K2 templates which are Bootstrap ready.