I've hit a lens wall. I initially bought the Sigma 17-50mm f/2.8. Foolishly I sent it back and decided to "upgrade" to a professional lens, the Canon 24-70mm f/2.8. It was foolish because the price difference was HUGE, $600 vs $1300, and I'm not seeing a difference in image quality, actually I think the quality isn't there.
The Canon lens arrived the other day, but it didn't work right. This was probably due to the large box it was packed in, and the big bubble to protect it from being jarred in transit was flat. I'm assuming it banged around and shifted more than a lens should, so needless to say, back it went. The replacement lens has arrived. This one works correctly, but I'm not seeing the image quality I expect from a lens more than twice the cost of the Sigma. If this doesn't change over the next few days, the Canon L lens will go back, and I'll happily welcome back the Sigma!
I think this also confirms when it's time to purchase the 70-200mm f/2.8, I'll be purchasing the Sigma lens for $1300 and forego the Canon for $2300. In many of the reviews it is comparable, if not better in regards to image quality. One last note, I didn't see chromatic aberrations in the Sigma lens, sadly, I've seen some in the Canon lens....
I guess I will not be a die hard Canon L lens fan girl. Sorry Canon, but it appears Sigma has some great lenses amongst their lineup!
For the pixel peepers who would like to compare. Here is an image from the Canon and an image from the Sigma.
Canon 70mm, ISO 100, f/2.8, 1/80
Sigma 50mm, ISO 200, f/2.8, 1/40