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.
Rather than hijack this thread further, I've started a new thread to try to answer your question about my 24-70 f/4 near MFD ...