Whilst the Sigma performed really well on the K100D I was never really happy with it after upgrading to the K10D. The clarity and colour seemed to change and was unimpressive. I only used it when I really had to. As such when I again upgraded last year to the K5iis I never even put the Sigma 70-300 on the camera.
Well I must say the the 55-300WR on the K5iis is a massive improvement and it was great to see the results. I like the colour, focus speed and balance. Overall I am impressed with this lens and it is more than adequate for my uses.
Here are some examples.