I appreciate the discussion from everyone. I think this lens is going to be a little polarizing because Canon is moving the STM technology out of the EF-S/EOS M sphere and into full frame lenses (most will agree that the 40mm Pancake was a different story for a lot of reasons).
I'm actually happy to hear the 24-70 f/4L have some ardent supporters. Frankly, I hadn't seen evidence of too many of them! For me, however, this is the biggest reason why the 24-70 f/4L still doesn't make a ton of sense:
I have to say that the tests (not necessarily his comments or star ratings at the end by any means, but all of the data in his plots) at photozone.de far more often match my own carefully tested findings than what I see at TDP.
I've used two copies of 24-70 f/4 IS and about five of 24-105L and the photozone.de results are closer to what I saw. I was never satisfied with the 24-105L near 24mm for finely detailed edge to edge FF landscape work, but I had no issue with the 24-70 f/4 IS (although the 24 1.4 II and 24-70 II were a bit better). And the 24-70 f/4 IS shots of fine branches against clouds and such were so much freer of nasty longitudinal CA (never mind better lateral CA, although this is a bit more easily corrected) than the 24-105 L real world test shots.
I will say that the first 24-70 f/4 IS I tried, while better than the 24-105L, definitely was not as sharp anywhere in the border regions as the second copy, noticeable difference. So lens copy to copy variation is real. The first one also suffered more in the mid-range than the second copy (still not worse than the 24-105L, but maybe not any better at all for the first copy). So some may depend upon whether you get a poor copy or a really nice copy. From what I saw from my own careful tests (25' target, indoors with constant lighting, 6+ manual 10x liveview focused trials at each stage, refocused for mid-frame and edges) and real world snaps of tricky forest scenes my impression was that a good copy of the 24-70 f/4 IS is just simply quite noticeably, considerably, better at 24mm than the 24-105L. (Although I looked at it less, I also felt the same for 70mm, in fact the top 24-70 f/4 IS I tried actually did better FF 70mm edges than any 24-70 II I've tried (And that's a number of copies) although it wasn't quite ever with the biting mico-contrast center frame of teh 24-70 II and it's a bit more like the 24-105L micro-contrast there).
Count me as another happy user of the 24-70 f/4L IS. As I've said in another thread, my copy was excellent at 24 and 70 out of the box, but disappointing at 35 and downright poor at 50 ... my sister's 24-105L was definitely better at 35 and 50. However, after sending my lens back to Canon for calibration, I'll happily pit my 24-70 4L against a 24-105L at any focal length. As has already been pointed out to my by a couple of CR users, Roger Cicala has commented the 24-704L has a lot of adjustable parts (more than most lenses), and it does seem the 24-70 4L suffers from substantial copy to copy variation. Get one properly calibrated though, and I think it is a very good lens.
My brother has the Tamron 24-70 2.8 VR so I will have to do some comparisons if I can find some time. I did some very quick comparisons just after I got my lens calibrated and came away with the feeling my 24-70 4L was at least as sharp from f/4 (I was probably concentrating most on centre sharpness), and its focussing was quicker and more consistent than the Tamron. That said, my testing to-date was pretty limited (and the possibility of new lens owner bias cannot be ignored! ) so I wouldn't draw any conclusions from it.
I noticed this sentence in Dustin's review:
"The trend of many lenses that Canon has offered in the past few years have been lenses that didn’t necessarily wow people on paper (or thrill them with their announcement) but have proven to be extremely competent lenses that have won people over on their merits."
I have to say I agree with that. I wasn't that excited about the 40 2.8 or the 35 2 IS but now own and like both, and looking beyond lenses I think the same sentiment applies to the 6D. I think perhaps the 24-70 4L might end up being another lens to which the sentiment applies, if Canon can find a way to reduce copy variation and get the "average" 24-70 4L performing closer to its best performance.
Anyway, Dustin, thanks for the review of the 24-105 3.5-5.6. Interesting to get your take on a consumer-grade lens after reading your reviews of high end gear. The new 24-105 sounds like it could be a good lens for a lot of people, even if not many of the people who haunt places like CR.
How does your 24-70 do at close focusing distance, particularly at the long end? Mine REALLY seems to struggle. It's fine until I get near MFD and then the picture totally softens up...
I find that the 24-105 is still noticeably better at 50mm though. It has gone back to being my default lens.