After a month or two with this extender I feel qualified enough to write a small review.
Overall very pleased with this extender. Currently i can only use it with my EF70-200 f/4. It drops a stop down to f/5.6 but it the lens is still very usable at this aperture.
This shot:
was taken in the rain on an Eos 5D EF70-200 f/5.6 1/320 iso 500 Focal length was 280mm.
I’ve found since purchase it tends to stay on my 70-200mm as I find the extra focal length better than losing it at the wider end. It compliments my existing 24-105 f/4 L IS as the wide end is 98mm. It should come in handy when I eventually purchase the 300mm f/4 L IS
Conclusion:
Superb accessory. No noticable loss of quality, a relatively low(ish) cost your get an extra .4 of a lens.