I have been on the fence about this lens for a while. I just ordered one today from BH photo. Here is a review from imaging resource
They claim it is sharper on a crop body which is what I will be using it on. So you are getting similar focal range and performance as you would from the canon 200-400 on full frame. The 1 stop aperature advantage should in theory help counteract the better iso of the full frame setup and you are talking about 1/3rd the price . I will post more when it gets here.
On a crop body, you will loose a stop in iso performance and an effective stop of DOF against full frame. While your camera will state f2.8, the reality is that it is effectively dropping a stop to become a 192-380mm f4..so it doesn't really gain anything. The AF on this lens slower and less accurate than the big white and the focal length drop as the focus pulls into MFD is really quite apparent and dissapointing. I found that I was better off with a native 300mm f2.8 lens or a 70-200 f2.8 LIS with a 1.4x TC.