I am using this with a 5D full frame dSLR.
My initial test shots suggest that the IQ when used with the 300mm f4 L IS is not noticeably decreased (viewing at 100%). That suggests that the prime lens on its own is outresolving the sensor, and that adding the convertor (which will reduce resolving power), does not reduce the actual total optical resolution below that of the sensor.
With this lens the AF and IS still work perfectly.
It also works well with my 70-200mm f4 L zoom, though I would only use it for part of its range, from about 140 - 200 as marked (so effectively 200 - 280mm) to avoid duplicating focal lengths. There would be litle point in using it at a marked 100mm (effective 140mm) with the convertor, as that
is part of the native range of the lens - why intentionally reduce the aperture and (theoretical) resolution?
Someone below comments on its use with the 100mm f2.8 USM macro. I would point out that it does not fit that lens, as the protruding front element of the convertor will not fit in the lens mount recess of the macro lens - so don't try it!