After being able to play with this new device for a little bit I am now ready to give my opinion on it.
First off losing 2 stops of light is difficult for any photographer, but the extra reach you can get with it on bright sunny days is definitely worth it. When out chasing birds or wildlife during the day and you wish you had just that little more reach the EC 20 gives you what you need for sure.
I have found a few problems with using it however. When coupled with the Bigma (Sigma 50 - 500 f4 - 6.3) it can be very useful but as you approach the end of the zoom length the focus is very hard to control. The slightest adjustment makes a HUGE difference in what you see and with the 2 stops light loss the AF has a really hard time grabbing the focus as well. For a wildlife shooter if all you really want is an ID shot at max zoom you can achieve that easily. After about 750mm (combined focal length) the focus remains soft and hard to control. Having to bump the ISO to get the shot in lower light situations leads to noisy and soft focused images. Still getting the extra 50% reach with a sharp focus is nothing to complain about.
Using it with the kit 40 - 150F 3.5 - 4.5 lens is a very nice combination. The AF works nicely at this level leaving your "Wide open aperture" setting at F8. No need to bump the ISO now so shooting at max zoom (300mm combined focal length) is a breeze due to the weight factor alone. Having a 300 MM lens that weighs next to nothing (less than 2 lbs combined weight) compared to the Bigma's 9 lbs almost left me feeling like something was missing.
The AF speed is uncompromized by the Teleconverter regardless of which lens I tried it with but the AF accuracy works better with the shorter faster lenses. The higher the aperture number is the less accurate and less sharp the focus seems to be with all lenses tested.
Sample shot are taken with the 40 - 150 kit lens at F9 ISO 200 first shot with and second shot without the EC 20 attached.