I had heard all of the negative things about this lens before I bought it. Knowing how picky photographers tend to be, I decided to give it a shot anyway. Well, everything they're saying is true. This lens does suffer from CA, and is not as sharp as an L lens. But guess what: It's NOT an L lens. If it was, it'd cost $2,000.
The IS on this lens works well. The range, as far as I can tell, is unmatched by any other lens out there when it comes to wide angle. If you're going on a vacation where you're likely to be taking photos both indoors and out, in locations where flash and tripods are not permitted (for example, every single tourist spot ever created), then this lens is your best choice, unless you don't mind carrying and swapping out extra lenses.
If you're expecting the kind of sharpness you get from a prime lens or one of Canon's L lenses, then forget it. It's sharper than the kit lens (18-55), and produces a nice 8x10 print, but it isn't going to please people who've been afflicted with L-itis.
Chromatic aberration, particularly purple fringing, can be problematic at the wider end, and is even visible on an 8x10 print in extreme conditions (such as dark branches against a light sky). But CA can appear on any lens, and the only way to minimize it is to pay more for specially designed or treated optics. And, once again, the CA isn't much worse than you'd get from the kit lens. Of course the kit lens is a fraction of the cost, but it doesn't have nearly the range, nor does it have IS.
Taking into account the range of the lens, the build quality of the lens, and particularly the Image Stabilization (which generally adds several hundred dollars to the price of a lens), the pricing of this lens falls into line with the rest of Canon's offerings. It may not offer the quality that people want from a $550 lens, but they're forgetting just how versatile this lens is designed to be. This lens isn't a deal, but it isn't the overpriced piece of junk that others are making it out to be either.
If you want razor sharpness and minimal CA, then go with multiple prime or L lenses. If you don't have the kind of money that this requires, or want the ability to not have to keep swapping your lenses to avoid missing shots, then this lens is really your only option. Be thankful Canon offers it.