I been using the 70-200 f2.8 IS
for a about a month now on a Canon 20D body and find the quality is excellent, and with the 1.6x magnification on the Canon sensor it gives 320mm focal length which meets most of my needs.
Although, I am thinking of getting a 1.4x converter just to give me that extra pull when needed.
Does anyone have any experience of this combination? Is there much loss of quality?