I have used the Canon 400DOII for a year now with 1.4tc and 2.0tc, coming from the DA560.
It's not as simple as dissing diffractive optics or praising them. The DO and PF lenses have strengths as well as weaknesses, but the same was true for the DA560.
In some ways, the DA560 is superior to the 400DOII, and in some ways it is inferior. So it all depends on which strengths you are after in a lens.
For distant rendering in harsh daylight, the DA560 completely crushes the 400DOII+1.4TC. But for waterfowl in normal light, the DOII crushes the DA560, that has a harsh OOF rendering of patterned water, and a truly ugly purple/cyan fringing in the bokeh that is impossible to remove in post.
Color is also a bit better with the 400DOII, less bold and more sophisticated, and also very fine detail is rendered better, where the DA560 excells in plain sharpness and clarity.
And of course, AF acquisition and reliability is of another level alltogether.
All these things way heavily when judging the useability of a lens, and you cannot dismiss diffractive optics with one or another horrible sample of its weakest point. Also, keep cost in mind, going to the supertele focal length is very costly, and brings with it considerable weight. Diffractive optics have no use in wide angle lenses, but in 400 or 500 mm lenses it is a very valuable addition to the lens offerings.
Originally posted by beholder3