I got a TC-20E in today as well (cheapest AF-I/S teleconverter... LOL).* While many have expressed that this combination is the less than ideal... well, it's what I can afford.* Most reports I've read have said that besides the f/5.6 effective aperture, the corners suffer; I find it unlikely that I'll be needing the corners at 400mm, so I figured I could live with it.* The loss of sharpness isn't horrendous, admitedly.* It's certainly workable for me until I upgrade to the big glass (likely to be years from now).
140mm, wide open @ f/5.6. ISO 1600
400mm, stopped down one @ f/8. ISO 200.
I hear the TC-17E II is a much better option, and even the TC-20E II is supposed to be slighty improved despite sharing the optical formula, but those cost more. Again, more on this item as I actually put it through rounds.