I received the lens yesterday and have been testing it in all the ways I can imagine.
The auto-focus works great, it's fast and accurate. The sharpness is outstanding, got super sharp images at very low speeds. I compared the images against a lens without the Image Stabilizer and the difference is incredible . I was holding the camera very carefully while using the lens without the IS and then I took the same picture with IS and holding the camera with shaking hands intentionally. The pictures with IS were much better, awesome feature. The zoom is perfect for mid ranged shoots, I guess I will need a larger one for shooting further objectives.
I made some moon pictures too, the full zoom deployed covered an area of a bit more than 3 x 4,5 degrees of sky. The moon appeared a bit small in the pictures but after processing the images they turn to be very good, with the main craters very sharp and with good contrast. I will keep making astronomical pictures with this product, I don't doubt there will be great satisfactions with deep space objects and long exposure pictures.
The body of the zoom is light weighted, I was very surprised by this because at first I thought it could be something to distrust but the quality is not affected by it and it's even a good thing since it's easier to handle it.
It can't be easier to use it, there is no need to learn, just point and shot.
All tests done with a EOS Digital Rebel XTi.