Conclusion: rarely does something live up to such a lofty reputation as this lens has among users. But at least in my case, this lens certainly does. I have had it for about a week, and put it through both lens testing and many different real world shooting situations, and this lens is just stunning in it's ability to deliver exceedingly sharp, contrasty pictures, even wide open. And if you shoot wide open, especially at the long end, you will see some excellent bokeh performance!
Pros: Super-sharp images, Superior build quality, Fast auto-focus, reasonable size and weight
Conclusion: This lens is simply amazing! It is tack sharp, not too heavy, and doesn't look out-of-place on a Rebel-sized body. The only potential downside for buyers would be the price; but it is offset by being an exceptional zoom lens that very nearly matches (and sometimes outperforms) primes and by the fact that it offers exceptional image stabilization. Canon claims up to 4 stops, but your mileage will vary somewhat (depending on how stable/shaky you hold a camera, etc.).
Pros: Super-sharp images, Fast auto-focus, you will get noticed (color/length), excellent build quality; not too heavy, excellent image stabilization, image quality on par with other canon primes
Cons: you will get noticed (color/length), a tad pricey (until you take a picture!)
Summary: OK, I was skeptical. An L lens is not exactly easy on the pocket. I spent a lot of time thinking about what L lens to get. I looked at all the reviews online and for the money, this seemed like the sharpest lens for the money in the Canon line-up. Honestly, I have quite a few lenses in the consumer and high end consumer class. They all looked excellent in my mind when I put them on my XSI. My most used lenses are a 28-135mm IS, a really nice 80-200mm and a 70-300mm IS.
Summary: This lens was recommended to me by a friend, so I already knew it was capable. The extras, though, make me smile every time I pick up the camera. The soft neoprene covering keeps the lens safe, is warm and comfortable in my hands, and takes care of my objection to the "white" lens. I got the plain black, and it looks very classy. The hood covering means I don't have to remove the hood in order to protect the lens. And it's all very light-weight!
Conclusion: I'm new to DSLR cameras & I was apprehensive buying a lens that costs more than some peoples cars(including ones I've owned). After reading 100's of reviews & researching other lenses I decided on this one. Let's just say that in 15min I took such incredible pictures of my daughter I looked & felt like a pro. that I can't think of a reason not to buy this lens. It's a plus that for the $ you pay that Canon includes a pouch & a hood with the lens.
Pros: Super-sharp images, Superior build quality, Fast auto-focus, looks cool on your camera, comes with pouch & hood