I bought this lens after much deliberation, angst, and help/adivce from fellow FM Forum users.
This lens in my opinion is incredible. A dream to use, it is fast, responsive, and is the perfect lens for anyone who looks for a long telephoto without the weight issues normally encountered.
In about one month of use I have found the sharpness, contrast and bokeh to be superb. Some people talked about suceptibility to flare and specular highlights. I have not seen a single case of this yet in my shooting.
I will not argue with those who say it is not quite as good as the 300 2.8. It may not be, but this lens is such a treat to use in the field, and the shots are so nice, I personally could not be happier with my decision.
If you can live with a 300mm lens, by all means, get the less expensive, and optically superior 300 2.8L. But if like me, you want one telephoto that is both long enough to capture birds and wildlife and l can be easily transported or carried in the field, there is nothing that comes close to this lens.
I guess my best endorsement for the DO technology would be that if and when Canon ever comes out with a 600 DO, I will be saving my pennies and nickles again.