I have used this lens with my Nikon D70 for about a month and taken about 200 photos with it.
The optical performance has surpassed my expectations. The lens is generally very sharp and only slightly soft at the corners at maximum telephoto (270mm) and full aperture (f6.3). Many photographers may not notice this unless they look carefully. At the other end of the scale (18mm) there is some barrel distortion which should be correctable in photoshop (though I have not done this). Also I think I see slight vignetting (I guess half a stop) at 18mm, but this may be due to the uv filter I use. Again, no serious problem.
However, there are handling issues. The most annoying is that autofocus does not always "confirm", even in good light, which means that the shutter will not fire as expected. This happens about 10-15% of the time and could be a show-stopper for some. The solution is to switch quickly to manual focus after the autofocus has done its best. I believe that this autofocus issue is worse if the camera battery is at less than half charge. Combined with a rather average focusing speed and mediocre handling this means that I would not recommend this lens for action/sports photography.
Another lesser handling problem is that the zoom control is not smooth and is distinctly stiffer in the mid-range (70mm to 130mm approx.) than at the ends. This is irritating and reminds me that I did not buy a Rolls Royce (or even a Nikon).
The lens is also prone to "zoom creep" - in other words it opens under its own weight while being carried. I don't find this a problem as there is a lock which keeps the lens at 18mm.
The vibration control (anti camera-shake)works very well and I can handhold 270mm telephoto at 1/100 sec., or possibly better. You can't do this with the Nikon equivalent lens as it doesn't go to 270mm!
Overall I am satisfied with this lens and it fulfills its all-purpose function well.