I was excited about this lens and really was looking forward to getting it and selling my 300/2.8L IS. However, I must admit I was disappointed. I did a number of comparison shots (inside and outside) with the 400 DO vs 300/2.8L IS with the 1.4x TC. I compared the shots on a tripod and a few hand held ones. In all of my comparisons, the 300 plus TC resulted in sharper and more contrasty images. While the 400 is lighter, it is not a small or lightweight lens. Therefore, it was not an easy lens to carry around for long periods. I personally believe this lens is over priced for the quality. If the lens was in the 2500 dollar range, I think it would be a reasonable value. Nonetheless, in the end I returned the DO lens.
Update: Based on other posts related to "sharper" copies, I did get a new copy to try and was impressed. While still not as sharp as the 300+TC, it was much much closer and overall the optics on the new lens was night vs day compared to my old one. As a result I am keeping this one and sold my 300/2.8L IS. I think there are others who have found many of the older DO lenses to have QC issues but I can say that the one i have is really decent and a keeper.