I'm surprised at all of the 5/5 reviews here. I shot side by side comparisons of the 70-300 DO, the 70-200 f2.8 and the 100-400 f2.8 lens. There really isn't any comparison in the quality of the images, the 70-300 DO is lacking in sharpness. I also saw some strange things going on in certain areas with color clarity and accuracy. While it still delivers a nice image, you lose some quality.
The only reason I bought this lens is to have handy when I'm trekking around shooting mainly wide to medium angle stuff. I'm usually carring a 16-40, and 28-70 as well as a 100mm macro and a a TS lens. I like this lens because its so light and small I can carry it, just in case I run into something that requires more than 100mm.
I will still use the other two when doing telephoto work ... they really are far superior.