They're excellent. I'd never thought of using the 100-400 II L for macro. Must give it a try.
At 400mm and MFD you get a .31x maximum magnification which is pretty awesome. Adding a 1.4x extender will magnify it even more. The compression at that length along with that close MFD gives ridiculously smooth backgrounds.
It's close focusing abilities is one of the 1-4mk2's best attributes.