Summary: Great for low light, natural light photos. Sharp photos even wide open at F2.8. The glass is L grade but the build of the lens is not. Common to all Canon consumer grade lenses. Overall I am very happy with the lens, especially at the price I paid!
Conclusion: This lens, even with the EF-S format and inferior build quality, produces surprisingly good images, outcompeting even such "L" lens as 24-105/4 L IS USM. Particularly related to chromatic aberrations which in this lens are neglegible.
Conclusion: I've hade this lens on my second body (7D) for nearly one year and I still think that's the best APS-C standard zoom on the market, as far as optical performances and focal range are concerned. Distortion at shortest focals is an issue and corner sharpness wide open is not impressive at all, but that's to be expected considering the kind of lens. What you wouldn't expect from such a lens, considering its price, is the poor built quality.
Conclusion: This lens is really great, it's super fast and quiet even in low lighting. Portrait are beautiful less noise then my old lens. I fell in love with this lens after upgrading it from my EF-S 18-200mm f/3.5-5.6 IS. It doesn't have the zoom range as the EF-S 18-200mm and a lock, but it's lighter. The build quality is bit similar. The EF-s15-55 get really dusty inside easily and fast though which I don't like. I would love to see an update of this lens. L build quality.
Pros: Fast auto-focus, Super-sharp images, quiet, light, low lighting
Summary: I bought this because it was advertised as a perfect copy with no dust on or inside the lens elements. It seems like a decent fast, sharp lens and the image stabilization works. The design of the lens is of the type that telescopes the lens when zooming, and in that process, a partial vacuum is created which sucks in air. In fact, when viewed in sunlight, I saw some dust specs. Probably not enough to worry about.
Summary: Takes fantastic "L" quality shots. I can finally get wide shots up close. The only downside is the build quality. It isn't top of the line for ruggedness. I've heard that dust can get I side the lens, but I haven't had any issues myself. Overall I'm extremely happy with this lens. The best wide angle zoom for a crop camera. The 17-40L is a little cheaper and built better, but is only f4, and the 16-35L is way too expensive.
Conclusion: Having used this lens for over a year as a primary go to lens I find that it has it's place . i also own the 70-200mm f2,8is mk 1 i use this lens every chance i can. you have to compare apples for apples... the dif in a L lens well what can you say it's a L lens ....that being said this lens has nice weight and balance the manual focus ring is quite sensitive but after repeated use its managable The auto focus is fast enough to perform as built.
Pros: Fast auto-focus, Super-sharp images, works well in dim light
Cons: lack of resitance of focus ring, lack of weather resistance