Only got this lens recently, but I can immediately tell that - optically at least - its quite superb. I'd give it a 10 in this respect. Good sharpness across the whole frame at all apertures and center sharpness really top notch even wide open. Center sharpness at f/2.8 is virtually indistiguishable from f/8, which is quite an achievement. No real need to think about stopping down to improve sharpness.
The colours are strong and images are nice and contrasty. Chromatic Aberration is less than I expect and not really noticeable. The same goes for distortion. I expect this lens to be slightly worse than the Tamron 17-50 in these areas, but on the contrary I have found it to be better.
What lets the lens down is the overly high price, and the build quality, given that price. If you buy a lens hood to go with it, it works out at roughly £700 in the UK; vs £300 or less for the Tamron 17-50, which includes a lenshood. Shamefully for Canon, the build quality on the Tamron is actually better and optically there's very little between them. Basically it boils down to going with the Tamron, or paying £400 more for the Canon with IS. This is hard to justify when Canon 17-85 has IS built in at a much lower price point.
At the price Canon are selling the 17-55 for, it should have much better build quality and be "L" specified. Or alternatively they should keep it as it is and drop the price by at least £200. £300 for the Tamron vs £500 for the Canon (with hood) would perhaps be fair.
So I think the 17-55 looks poor value for money. But still, it does take GREAT pics.