I believe that this product is wonderful in almost every way...
The only problem I have with it is the price, but I guess it's only reasonable.
The lense is very sharp. The ES image stabiliser works very quietly, the most silent of the lenses I've had so far. I used to use Canons' EF 500mm/4L IS USM and the EF 600mm/4L IS USM.
One of the largest advantages of the lense is that it's considably light. It also wonderfully works together with the 1D Mark III.
All in all, I'm very satisfied with this purchase.
Here are some links to picture examples done with this lense: