For starters: I have done through 2 copies. Result I sold the lens and bought 70-200 L F/2.8
Why sell? Softness was killing me. While this is an excellent build / look lens, the images produced on 20D and XT were just OK. OK is not a definition of the L standard. Now, I might have gotten two bad copies sure, but we are talking about the highly priced L optics here and bad copies are not acceptable. I will no rate this lens at a 2 or a 5 this is just no fair, but its not (not for me anyway) a 9 or 10 in my case. Taking pictures with IS ON gave me decent pictures AFTER the unsharp masking in PS this is unacceptable (for me). The F/4 is slow, but by all means is not the disadvantage of the lens. You know what you are buying why mark is as a con? Yes its slow, but would it be sharper Id keep this lens without thinking. Having taken a few dozens shots with 70-200 L F/2.8 (non-IS version) at 70mm F/4 (through to 105mm) and compared it with 24-105 F/4 (same: 70mm 105mm at F/4 with IS ON) served me as a confirmation that the 24-105mm F4 L lens is indeed soft. Yes stopping down does make the image more acceptable, but you are loosing the F stops and the low light shooting becomes a huge challenge even with IS ON. I know now that F/4 is just not my cup of tea and F/2.8 is indeed a solution for the type of photography. I might consider buying this lens again (for vacations) in the future, but only after the Canon QC deals with all the issues AND when the price drops (as I think the lens is overpriced).
I would still recommend the lens to someone, who would like to own only one lens that would do it all (providing you have the time for going through copies till you get the good one). The lens is very well constructed and has an incredibly useful rage. If you are looking for a walk around glass and not too picky on fine details this might be a winner in your books.