Chris I have this lens since 3 years. It is great, not too heavy and of a superb quality. I have used it on many occasions with both the 1.4 and the 2 time converters for my hobby: bird photography. The viewfinder gets a little dark with the 2 times though, and with the limited depth of field this can become a problem.
I have proof in many excellent pictures that the 400 mm is better than the 2.8/300 mm, which is a little soft. I have once had the 2.8/200 mm which is just as great as the 400 mm, but for my work (bird photography) not long enough. Since one year I have 4/600 mm as well. Bloody heavy (some 6 kg), brings a little more tele than the 400 mm and with converters it is magic. When you use a converter, picture quality is only top if you shut the aperture 2 or more stops. This limits using the two times: for a 4.5/400mm the converter brings it to f9 and you have to stop it down to eg f13 to get the best out of your lens. Lighting conditions will not always allow you to do this unless you use high speed (more grainy film). Although I can only load "smallish" kind of pictures (and in no way can really represent the quality of the lens optics), I have uploaded one that I made 2 years ago of a European goldfinch: at a distance of 10 meters from a shelter with a 1.4 time converter under good lighting conditions.
Anyhow, nice stuff but expensive. I have bought mine (demo almost new) for about 1500 Euro and have seen many of these lenses second hand in Tokyo photo stores in the last months for about 900-1100 Euro (in mint condition). I have no idea what the prices are where you live, but 1000 Euro/US$ seems like a decent price for a good condition 4.5/400 mm lens. Success.
Peter
