Congrats on the EC-20. It is a worthwhile addition to the 50-200mm lens.
Have fun, enjoy it and post a few pics when you've had a chance to use it.
I find that the image quality does degrade a tad when the 50-200mm is fully extended. And, of course, you know that you'll lose 2 stops on the aperture.
Under normal daylight conditions, I've not found losing 2 stops to be an issue at all.