OKay so Ive had the Sigma 18-50 mm f2.8 HSM for a couple of weeks now.
Ihaven't gotten to use it as much as I would like but Ive tried it out. So far I like the build quality, perhaps not as good as the 10-20 mm I had but still, better than budget Nikons. The f2.8 is very fun to play around with, I like the bokeh it produces. The autofocus is very fast and silent. The weight of the lens itself is about teh same as the 10-20, which is good as it gets my camera heavier so its easier to keep steady, and it feels better. One minor problem however is that the zoom ring is backwards from Nikon lenses, that is, you have to turn it to the right to to zoom out instead of left. Im guessing Ill get used to it though. Another minor annoyance is that the focusing ring rotates when it autofocuses so its very easy (if you have big hands like me) to be holding it when you focus, and then youll feel a tug at your fingers. I heard a very unpleasant sound when this happened so Im going to try the utmost not to let that happen again. I do however like the focusing ring on my 10-20mm much better, where its not in the way of my hand.
The picture quality as far as I can judge is very good. Sharpness at f4 and up is better than my Nikon lens. Colors and chromatic aberations are okay. the Macro is very good, I found that I could use my lens hood as stabilization when taking close ups. It focuses closer than what Sigma said it would (20cm) for some reason, more like 7cm. Its fun to play around with.
In short, Im happy with it, except for the focusing ring turning around and tugging at my hand. The light gain compared to my old nikon kit lens is about 2 stops faster I figured, zoomed all the way out. At 50 mm it would be far more of course (5.6 versus 2.
Bottom line, Im happy with it, except for focusing ring