I'll quickly chime in here regarding this lens. I've had it for two days, and I'm not terribly impressed so far. I was excited to get this sucker, as I'm looking to consolidate my collection of glass and this lens could replace several other lenses if it were good enough.
This is certainly a good modern lens: all the positive reviews can't be wrong or outright lies, can they?.. The samples I've seen posted all over the place look mostly good.. However, for $2049 and after reading all the over-the-top reviews I was expecting a bit more. That being said, I have in the past disliked certain lenses at first, only to learn their quirks and produce acceptable results after time.
I did not own the mark I version of this lens, but I have lots of others (albeit primes, different zoom ranges, IS included, larger apertures, tilt-shifts, etc.) to compare it to. Now, I'm not using Imatest like the lensrental.com guys, but my eyesight is quite good and I know how to use my 5dMkII and MkIII just fine
I'm fully aware that I'm not performing scientific tests here, and that the comparisons are in many cases completely unfair. Hopefully, I'm not flamed to Hell for this.
First off: aside from corner performance (where the 24-70 is quite good), the 24-105 f/4 IS seems nearly as good @f/4 all the way from 24-70. That in itself is bad news for such a new and pricey lens, and I was completely shocked. I tried hand held with IS on, and tripod mounted (IS off) shots and the results were fairly similar. The 24-70 did have better contrast than the 24-105. Perhaps my 24-105 is a real gem but something tells me it's as average as everyone else's. Next up, I compared a few quick landscape shots taken with it (at 24mm f/3.5) to the 24mm f/3.5 TS-E II. I felt the TS-E was clearly better. Again I was shocked, as the lensrentals.com guys said this lens bested the TS-E... perhaps only on paper. The next comparison wasn't exactly fair or balanced (lol) but I did it anyway.. I took a few quick shots of my wife (head and shoulders) with the 85mm f/1.2 II (@f/1.2) and compared to similar (yeah, I know.. it's pineapples vs. peaches) shots @70mm f/2.8 with the zoom. The in-focus areas produced by the 85 seemed sharper to me. Yes, it's a prime vs a zoom, yes it's 85mm.. yes it's f/1.2 vs f/2.8, but I was still expecting better from this new pricey beast. Lastly, I snapped on the 70-200 f/2.8 IS II, which I consider to be the best zoom lens I've ever owned. @70mm, f/2.8 (IS on or off) the 70-200 is again better than the 24-70II (@70mm f/2.8 ). I'm going to shoot with this sucker for a few days straight to see if I can get better results, but so far I consider this lens somewhat of a letdown.
In closing: I believe that most of the "bad copy" scuttlebutt I hear about certain lenses is nonsense (or at least the differences in resolution copy-to-copy are next to undetectable by the human eye), so if I find this lens to be a dud I'm not sure I'd exchange it for another... We'll see..