I have that combo (70-200 II and 2xIII). I can´t say I have used it a lot, but as you say, you can pack an extensive set of focal lengths in a small pack, so it is very handy. I brought the combo to Corsica and Provence this summer and used it to photograph some vultures in flight in Gorges de Verdon and a hawk and some large insects on Corsica. I had to crop most of the images, but they still look good. Resolution, color etc. are better than I actually expected when I bought them. AF will be a little less reliable, but still good enough (I used the 5DIII on both trips). I have used the extender more with the 400mm f28L IS II and that combo produces excellent results. I must admit though that I find the price a bit stiff for an extender, but what Canon product does not carry a stiff price these days ...