Conclusion: There seems to be a lot of quality control issues with this lens. The main problen i have with this lens is "zoom creep". Its also hard to focus manually. Other than that, the lens is good. I traded the kit lens that came with the camera for this lens, and i'm glad i did. Going by the reviews, it seems like there are a lot of lemons that get by quality control, so its a hit or miss with this lens. I would look elsewhere if i needed a reliable lens.
Conclusion: Excellent lens, when it works. The problem is I can't tell when it's going to work, and when it's not. It's almost moody. I stuck with Canon, and got 2 new lenses to replace it. Keeping it as a possible backup. I would not recommend it, especially at that price.
Cons: Slow auto-focus, Poor build quality, sometimes great sometimes a zero
Excerpt: I bought this lens along with a 30D the last week of March 2006. I had high hopes for this lens. In reality it is a failure. Canon should have recalled this lens and offered customers a new one in replacement. It failed twice. The first time I sent it to Canon where it was inspected and sent back to me. There were been issues with the auto focus and usm. It has since developed the problem with the aperture ribbon cable. It is a known defect with this lens.
Cons: Heavy, Lens Creep, Manufacturer Defects, Poor in Low Light
Conclusion: Took 11, 000 pictures with this lens, then suddenly it had a communications failure with t3i. this is a known defect with this lens -- i.e., the flexible cable inside the lens that does the electronic communication wears out and fails. The lens then stops working completely. The lens was bought in 2012 and failed after 18 months (out of warranty of course). That it failed on a trip to an african country was very disconcerting.
Excerpt: I have owned this lens now for 5 years. It is okay for general use and is way better than most kit lens for cropped sensor cameras. However, the lens creep and weight do not justify quality of photos from this lens. The lens failed while on a big trip to Turkey. Fortunately I had a prime lens to limp along. I was able to get it repaired when I arrived home from an [*] person. It now works fine.
Conclusion: The autofocus failed during a trip at the Everglades, I had to work on manual mode all the time. The repair is a very difficult task and costly. I has the lens for over two years, Canon should stop advertising this lens.