Forgive me for bringing this up again, but after being burned and frustrated by the 24-70 II, let me voice my opinion once more:
Just be careful when you purchase the lens, and make sure that you test it thoroughly to make sure it is working properly. It is my feeling, and I am sure some would back me up on this forum, that Canon is suffering from some serious quality control issues with this particular lens.
I, myself, experienced a severely decentered element on my first copy, clicking sounds from the zoom on my second copy, and another decentered element as well as squeaking on my third copy. It wasn't until my fourth attempt that I finally managed to get an acceptable lens.
I also want to mention that I ended up purchasing the lens twice. My first copy was sent into Canon after less than a month's use (unfortunately I passed the return date for the lens at the store) for the decentered element. Canon kept it for six weeks, twice as long as I had actually had it in my possession, only to return it to me and saying everything was in spec when it clearly was not. I called up Canon's Canada CPS and explained why I was not satisfied with the repair and the lady I spoke with told me Reikan FoCal could not be trusted and that my lens was properly looked at and adjusted by their technician. I told her she was wrong and she replied by telling me (her words) that I could "waste time sending it back in, but chances are the techs wouldn't do anything to it."
At this point I lost my nerve and went up one side of her and down the other about how awful her service and response was, and that this whole ordeal is unacceptable for such an expensive and brand new piece of equipment. Obviously, coming from her previous response, she still did not care and just said "send it in."
Having sold my trusty 24-105mm to pay for this first 24-70 I was kind of at a loss for having a decent walk-around lens. I was also determined to find a good copy of this lens so I went and bought my second copy from another retailer here in town. Thankfully this time I was cognizant of the problems too look for and returned it as many times possible until I was satisfied.
Unfortunately I managed to wrack up quite the nice credit card bill purchasing the lens a second time. I still own my first copy and am considering selling it at a massive loss, or sending back to Canon for repair and then trying to sell it. Either way, I've never been more frustrated with a product or the customer service of a company before.
As far as my working copy goes: the images are fantastic and incredibly sharp. It runs circles around the 24-105.