Excerpt: I've had the lens for 3 weeks and run a few tests. I've determined that the lens doesn't perform any better in low light then the Canon 18-135mm. In normal light the photos are sharper but the difference wasn't as significant as I expected when viewed at less then 100%. Overall, the lens does perform better then the 18-135mm, but I'm not sure if the difference justifies the price.
Pros: Durable, Easily Interchangeable, Full Time Manual Focus, Nice Bokeh, Strong Construction
Conclusion: This lens promises to be a great walk-around lens but I find the image quality to be lacking. And it has developed lens creep, such as when I shoot at 35mm and then tilt the camera down only a little to change settings by looking at the LCD on top of my 50D and 60D, it rotates on its own to 50mm when I bring it back up to look through the view screen. Also, its corner and edge image clarity at the short end (15-24mm) is poor, with lots of distortion, CA, and blur.
Conclusion: I use a 60D most of the time, but needed an extra camera while doing a wedding video, so I got a T2i with the 18-55 kit lens. This lens has a good overall zoom range, and it's good for somebody who only wants one lens, and doesn't want to spend much money. The downsides are that the images are not sharp. Even in the perfect light in perfect focus, the images are fuzzy compared to the other lenses I own.
Conclusion: I really love this lens with my 7D. BUT, was really disappointed when autofocus motor started to squeak after one and a half years! And unfortunately it's no longer under warranty so it will cost to get repaired. I don't use auto focus anymore because its so embarrassing having a 800 dollar lens squeak like crazy when shooting photos! This is an amazing lens but sadly didn't end up so good for me after 1 1/2 years of purchase.
Conclusion: With a focal length range equivalent to 24-136mm on a full frame camera, this lens could satisfy the majority of my needs. I bought a used copy, thinking that it had the potential to replace my 24-105L. The 24-105L is a great lens but is not always wide enough on a cropped sensor body. I was satisfied with the quality of construction on the EFS 15-85mm and could forgive the occasional lens creep but the optics didn't measure-up. My copy was soft, particularly so at 35mm.
Excerpt: I just bought this lens to use on my T2i, and I sadly will be returning it. The quality of the shot (when it cooperated) was nice. But it had a lot of focusing issues. Either it would take too long to focus or it wouldn't focus at all. A lot of the time I was unable to take a picture using auto focus. The lens movement was very stiff also. Perhaps it was too new and needs to be 'broken' in. The weight of the lens was very uncomfortable.
Excerpt: This lens features the best all around focal length range of any canon lens. That being said I have serious doubts about it's quality of construction and durablity. This is the third one I have bought. Each one was sent back to the retailor except this one wich was sent back to Canon because of a loose element and ill functioning imange stabilizer. It would be hard to believe that I bought the only lemons that Canon ever made.
Pros: Easily Interchangeable, Great focal length range, Lightweight
Excerpt: This lens is okay. It's ideal for travel and general purpose, but it doesn't come close to the exceptional optical quality of my Canon 17-40 L zoom, which I've used as a general lens on APS-C sensored Canon cameras for many years. It's perhaps closer to the Canon 24-105mm L zoom I use on my full-frame Canon cameras, but not quite as good. Its okay, but not stellar, ie a long way off prime lenses such as my Canon 85mm f2 L or longer prime telephotos.