I'll try to keep this reasonably brief, but thought I would share.
First week of April I bought a 24-70 UY date code new. I had just bought a 70-200 F/2.8 and was blown away by the sharpness, even wide open. The 24-70 disappointed in both softness and chromatic aberration.
So, I sent it to Canon Irvine. They agreed it had a problem and adjusted, sent a cut and paste explanation and sent it back. It was better, but still not as good as the 70-200, or 24-105 for that matter, and I kept it for a while and tested and shot. Sent it back again. So soft and hard to focus wide open or even at F/3.5.
They found more problems (why didn't they find them the first time?). Sent it back, tested extensively and still not happy.
I escalated to Customer Relations. They would not exchange but wanted another opportunity to adjust. So back it went. This time for over two weeks and even got a call from the service manager. And back it came. Amazingly, they found the same problems as they found the second time. And had my "problem" mislisted on the work order as excessive back focusing, which was not my problem at all.
I took it on vacation after having to Micro Adjust out the excessive front focusing they put in. At F/5.6 or smaller, it was really good. Really. But again wide open, everything soft as can be. And I bought this lens for concert work so wide open is a requirement.
Got back to Customer Relations and escalated. The senior CR person finally agreed to exchange and I received a new lens today. To their credit, they came through although I wish the ordeal had been a bit less.
Night and day. This rivals my 70-200 and I can finally have confidence in it. It almost focuses in the dark. Softness gone.
So, this confirmed for me, especially on this model, there are good copies and bad copies. And also confirmed that some bad copies at least cannot be made good at Canon Irvine. In my opinion, they should have acknowledged the lens was not up to snuff earlier on. But I am pleased after all of this.
And as an aside, I bought this lens from J&R in April. I emailed them in July letting them know this was in no way their fault or problem, but as a large Canon dealer could they help me convince Canon to exchange it. They replied they would exchange it for me and if like new including packaging, no charge but freight. After 4 months!! Absolutely amazing customer service. If I had not been on the other side of the country and also feeling like Canon should own this problem, I would have taken them up on it. But they earned my future business with service like that. Not to say Adorama or B&H might not have handled it in a similar way, but this was over and above in my opinion.
So just thought I would share. I read so many threads on good copies, soft copies, is my lens sharp enough, you must be crazy, look at mine at 2.8, etc I had to chime in. There are bad copies. And sometimes service just cannot get them right.