I bought this for my Pentax K200 camera body and the resulting image quality is extremely good. At its first outing to test it I photographed Sanderling (birds), general landscapes and banger racing on a beach. It was January so the light was low. The results exceeded expectations.
The product itself is compact and light for the focal range and quality of images - ideal for travel. Okay, so you lose a few stops at f4.0-5.8 compared to far more expensive lenses but the trade off is worth it. I have also tried it in very low light and combined with modern digital technology, the resulting images even at f5.8 were more than acceptable. The body is as you would expect at this price point (£237) but looks robust enough for a reasonably careful user and holds well.
Being most experienced in manual focus, and only having purchased a couple of other auto-focus lenses in the past year, I have to say this is the slowest and the focal system did get confused from time-to-time requiring a refocus on a completely different object before returning to the original. Sometimes the targets were difficult so I will forgive this for the moment and see if my technique can improve on this flaw. In saying that, this did mean I missed opportunities. Notwithstanding this, when photographing racing cars on the beach, even at maximum zoom, focus lock on this was very quick and accurate resulting in some cracking images. If you know birds, Sanderling are a bit like very fast wind-up toys, and again, overall the focus was pretty good for undoubtedly a tricky number for the autofocus system to cope with.
This ocassional problem with focus is the only reason I do not give a 5 star rating but on the image quality, portability and price I would heartily recommend this lens. If you want maximum aperture control and are more likely to be static than mobile (eg in a hide), go for a more expensive fixed focal length lens. I have an old second-hand manual focus Tokina prime 300mm lens and the picture quality is tip-top, but I can see it languishing in the bag with the new 55-300mm pentax just being so much more practical. With familiarity I expect to improve on the focus flaws and get the most out of this lens.
I work on the basis that 5 star is absolutely perfect. I would award 4.5 star for this if I could.