Evaluation: 
Conclusion: Excellent all-around lens
Review: We bought this lens for a wedding occasion and it performed great!
The lens is very fast and very sharp. It is a little heavy and unbalances the k100D a little. I guess it would be a better fit for the k10 or k20. We love it and are amazed at the photographic opportunities that a faster lens provides. Spend the money... avoid the pentax version because it is not better optically according to many reviews.
Evaluation:
Conclusion: Great Lens for the Price
Review: I was initially thinking about the Pentax 16-50mm, but changed my mind due to the QC problems they are having with it. I did my research and many, many users highly recommend this one. I took the chance and could not be happier. This lens is fast and the IQ is as good or better than most other lenses.
Highly recommended.
Evaluation: 
Conclusion: Sharp lens for a budget price
Review: I have this lens for two weeks now and have taken more than 200 pictures with it using my Pentax k10d. My working lens so far has been the Pentax 28-105 f/3.2-4.5, so I opted for a wider angle. The choice was between the Pentax 16-50 f/2.8 or this Sigma 18-50 f/2.8. The cheaper price of the Sigma (60% of the Pentax) and relatively good reviews in magazines and on the net made me buy the Sigma.
The first impression is a solidly build lens with quite a heavily dampened zoom ring. The protruding front element had some tiny bit of radial play, however the packaging was slightly damaged on the plane home. Nevertheless, this has no effect on the image quality. The lens is very very sharp. Side-by-side comparison with the Pentax 28-105 shows a dramatic boost in sharpness across the image. Focusing is good and quick, however I had a chance to play with the Pentax 16-50, which focuses quicker due to the HSM in the lens. Shots taken during fishing last weekend reveal the slight distortion of the lens at 18mm, which can be easily corrected in photoshop. However, there is some very slight wave-type distortion, which is nowhere near noticeable in real life but becomes apparent upon close scrutiny in Photoshop after I corrected the first-order barrel distortion. I should stress that the wave-type distortion is very very small. Out-of-focus bokeh is very pleasant for this lens at wide apertures. The colors are vibrant and better than the results I had with the Pentax 28-105. Vignetting is generally not an issue and is only slightly visible at wide-open apertures and can be corrected using Photoshop.
Overall I am very happy with this lens and would buy it again. The cheap price is a major argument for this lens and also its sharpness. I find it is also a good lens to go with the sigma 10-20 without much overlap in zoom range. There are only minor points where it looses out such as the little slower focusing, therefore 4 out of 5 stars. Highly recommended.
Evaluation:
Conclusion: 18-50 Macro rules!
Review: I bought this lens to replace the crappy kit lens I got with my Pentax *ist DL2, and I've gotta say, there's no comparison. Granted there's quite a price differance between the two lenses, but you get what you pay for, that's for sure. This is my new walkaround lens, you won't be dissappointed.
Evaluation:
Conclusion: A great value lens for great value cameras
Review: If you're reading this, you've probably bought a Pentax camera like the K10D or K100D, or are thinking of doing so. Most likely, you were drawn to Pentax cameras because they offer a good price/performance ratio. You don't want to pay for the biggest names - you want a great camera with which you can take great photographs, on a reasonable budget.
This lens is the perfect accompaniment to that philosophy. It is not the best in the world, but it is the best you can get for $400, and for some way above that. It covers the range required for walkabout photography, and is reasonable convenient for that purpose, despite the wide body required by a 2.8 lens system. It also feels good in your hand and looks great on the front of your camera.
Buy this lens now (perhaps along with the Pentax DA 50-200mm f/4-5.6 ED) and make some great pictures.
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