This is the lens that I've wanted for the system. I often carry the 70-300L along with my Tamron 24-70 VC + 6D while traveling. I also bring the M for the moments when I want to go out very light, typically with the 22mm. Being able to leave the 70-300L home would be sweet; I use a tele sparingly for general travel, so as long as the image quality is pretty decent this should do the trick for me.
Somewhat surprised by the f/6.3 aperture; I've not known any Canon to have a maximum aperture smaller than f/5.6, although the practice is not unusual among 3rd parties.