This extender is exactly what I needed for my 70-200 and my 100-400 Canon lenses. Little loss of light, great clarity and speed. I would recomment this over the 2.o, which losses 2 F-stops in apeture. Two thumbs way up...
It is hard to get too excited about a converter, but the Canon extenders do their job well. With a 70-200 f/4 IS lens the 1.4X II extender showed no noticeable image degradation, handled nicely and maintained lens aperture and focusing functions. IS continued to work nicely as well.
Only downside is that it doesn't work with many of Canon's less expensive and EF-S lenses.