Summary: Clear and Bright

Comment: I bought this lens as a replacement for the kit lens (28-135 3.5-5.6) whereas the kit lens was very soft at the edges, this lens is crystal clear. I use it as my walk-around lens. With it's good wide angle and fast f2.8 low light capability this lens is one I can count on for most situatuions.

Summary: great all around lens

Comment: This is a great all around lens for the canon rebel xti. I had the kit lens but was not happy with the photo quality and especially for indoor photos there was not enough light entry to take a good quality image. This is a big step up. However the pop up flash will leave a lens shadow on your close up flash images due to the larger size of the lens, so you will need to get a separate flash (I got the 430EX) to fix this issue. The image stabilizer makes a big difference in image quality.

Summary: Big and heavy ... but the best you can get for cropped sensors

Comment: I've been using this lens for about 6 months on 40D in a variety of conditions. Fairly dusty hikes and light rain. This lens has not failed to impress me. Sharp and fast. And I'm quite spoiled: I've sold 17-85 as it did disappoint me a few times in several respects and I own some of the sharpest primes. Also it has virtually no chromatic aberrations or distortion.

The only significant drawback of it is the size/weight. It's just too big for casual photography. It does inevitably attract more attention than smaller and more compact zooms. Funnily most people expect it to be some sort of 20x superzoom and can't believe that it's just about 3x zoom. Also it's long length makes it a bit problematic to use with flash. When extended it creates a large shadow on the picture.

It is also rather expensive ... but once you put it to some use you'll not regret the money.
Finally I'd be certainly happier if Canon has included the hood. A $2 piece of plastic that they charge $30-40 when bought separately.

Summary: Dust !!!

Comment: First the positives. The lens optically is a good lens, it does sharp images, nice colors and all that stuff. But the big problem is dust that gets inside of the lens. After 9 months of usage there is so much dust inside that it makes the lens basically unusable. Do not undervalue this problem like I did. Or make sure you may eventually return the lens if there is a problem. The lens already came with 3 dust dots inside which wasn't such a big deal but you should see it now. I have 70-200 and 24-70 both L line there is no dust problem. There is no dust problem even in my kit lenses. I am surprised that Canon can release such lens, because this is not a cheap lens.
At the beginning I could not decide whether to get 16-35 f2.8 II L or this lens. But Since I have Canon D 30 I preferred to get wider lens. No I would go for 16-35 even at the cost of lower zoom range and extra cost.
Now I have to figure out how to return the lens, I bought it in the US from BH photo, but now I am in Europe, so it is a bit complicated.

Summary: Pricey, but worth it.

Comment: I bought this lens to use with the Canon 40D. It covers the wide end very well. The IS is fantastic; you can shoot hand-held down railroad tracks from the passenger side of the car while crossing them! AF is fast and quiet, and f/2.8 allows you to use this lens in almost any situation. Image quality is outstanding. Colors are vivid, images are super sharp, chromatics are imperceptible. I have no vignetting at all wide open.

Downside: size, weight, and cost. The size and weight are insignificant when the image quality is taken into account. The cost, while high, is in line with other lenses of this quality. If you want the best wide angle zoom lens for your xxD or Rebel, this is it.