I have this lens for about 2 months now and I can say that this may be the best value lens. The image quality is actually very good. I'm not a professional but in last one year, I have tried a lot of highly rated lenses and I will say that with respect to image quality, this is surprisingly very good.
I had the highly regarded 70-200mm f/4 L lens and I will go on a limb to say that with respect to image quality and sharpyness, I can "almost" compare to the L lens with added IS which actually really works.
The one problem I noted with the lens is that there is some kind of jitter when you press the shutter. I'm not sure if this is how IS works or it is something else; but that results in focus shift a lot of times between when you press the shutter half-way to when you press it fully. If you wait for a split second and then press the shutter fully, the focus remains good.
Otherwise, highly recommended lens. Actually I compared the pics at 85mm with my 85mm prime lens and I was surprised at the quality. Of course, the prime lens just rocks when it comes to focusing.