Summary: I would definitely compare this to OEM lens at higher prices. Got rid of my 18-70 and 55-200mm lens so I don't have to change lens often. I thought I would be un happy with the slower speed at 18mm compared to other lens, but surprisingly I have not had any issues with the speed of the lens. Appears and feels like good quality. Has some lens creep, but that is expected.
Summary: buy this product for the fact that I had in my possession two goals, one 18-55 and one 55-200. I take this to rearrange a lot of work for each other and buying the Sigma 18-200 simplify these step!