This is a professional grade lens. People do expect miracles out of something at this cost, and with this "prestige". I am not aware of any particular issues with the Canon 24-70mm f/2.8 L MkI that would cause more copies then the average to have issues. One way of dealing with said "issues" is to send in your camera bodies along with the lenses that you want to match to it to Canon. They actually can perform a series of adjustments to match the lenses that you have to the bodies that you wish. You can do some of this by using the Micro-adjust feature of some of the current Canon DSLR bodies.
I think you would benefit from reading an article from the LensRentals.com CEO - Roger Cicala. He is very experienced in the sense that he gets a very wide perspective into a huge range of lenses. His article "This Lens Is Soft" And Other Myths provides a great deal of information on the topic and really helps to dispel the myths that you commonly can see in internet forums.
While considering all of the above, you can still perform tests on your equipment to make sure they are fairly accurate to begin with. The existing question that I asked goes into great detail on this: How can I test a new lens to make sure it is operating correctly?
This question also gives more information on micro-adjusting: What is the best way to micro-adjust a lens?
Overall, I would just pick up a copy, and give it a few quick tests - if the quality is up to your standards, there is no reason why you should worry about what others have complained about. Canon doesn't produce many lenses that have common issues(but a few do exist), and this isn't one of them.