When I first started searching for a wide angle multipurpose zoom lens, I shortlisted Canon 18-200mm IS, Canon 18-55mm IS, Canon 17-50mm F2.8 USM, Sigma 17-50mm F2.8 & Tamron 17-50mm F2.8. I read various expert reviews, user reviews and lens comparisons on the internet. Despite very good reviews, Canon 17-50mm F2.8 USM was striked out as it did not fit the pocket. Canon 18-200mm IS & Canon 18-55mm IS were rated average and inferior to any constant f2.8 spec lens in low light conditions. Though, there were various comments w.r.t. non Canon lens like they have slow Autofocus and are noisy, in the end I decided to go for a Tamron 17-50mm F2.8 for its constant f number. The model didnot support Optical Image Stabilisation, but it was the best bargain for my moderate budget. There were certain performance related issues listed in the forums with non-Japan manufactured lens, so I purchased a Made in Japan piece.
My experience with the lens is mixed. The build is sturdy and feels good to hold. The zoom ring is a bit stiff and it has not eased with use. The lens is excellent in good light conditions and give sharp photos. However, in low light conditions it struggles with autofocus. At 17mm / f2.8, even with manual focus in low light conditions, it is very difficult to get a properly focussed photo. At 17mm, barrel distortion is noticeable. From f3.5 / 24mm onwards, the results are consistently sharp. With crop factor of 1.6 for my Canon 1000D, focal length of 24 mm actually translates to 38mm and it doesnot remain a wide angle lens in enclosed spaces with poor lighting, as one would want to. Similarly at f2.8, the photos are not consistently sharp and so I avoid f2.8, the very reason that guided me to buy the lens. The autofocus is a bit noisy and slow, however the Canon counterpart with Image Stabilisation and USM motor is 3 times costlier, so no regrets. Initially I struggled with this lens. However, now I have adopted to it and use it with confidence.