When the 6D was announced, a lot of people would have opted for the 5D II instead, but then the 6D's IQ and other features won out in the end. When the 24-70 f/4 IS was announced, a lot of people swore by their 24-105 f/4 IS, but now people are considering the 24-70 f/4 IS over the 24-105 f/4 IS.
+1. Dead on. In my hands and for my needs, The 24-70 F/4L IS is a comprehensive winner. It's lighter and shorter than all the L 24-something lenses, sharper than everything but the 24-70 F/2.8L II, and the 0.7x macro is unbelievably handy in walkaround shooting situations.
Comparing to my previous 24-70, the older 24-70 F/2.8L I, I gave up one stop in return for IS, sharper performance, a massive weight reduction, useful size reduction (I can get the F/4 into some sporting venues with a 6" size limit), and the macro is just gravy. That's a terrific trade for how and what I shoot.
There's a long-running gun battle between 24-70 F/4L IS proponents and 24-105 F/4L IS proponents, and I don't want to start the length vs. overall IQ/features debate here as people have really strong opinions about this. Suffice it to say, you know where my vote is.
I see the 16-35 f/4 IS ultimately taking over the spot of the 17-40 and will create a trio of L f/4 IS zooms: 16-35/24-70/70-200. There is still room for a 16-35 f/2.8 III and a 14-24 f/2.8.
Agree again. That seems to be where this is going -- Canon is pushing high quality F/4 zooms that are worthy of the L designation, but do not have the enormous / heavy / 82mm filter / built-for-war designs that the top of the line F/2.8 zooms represent. I'm tickled pink about that as I get a 9/10 lens at everything except for aperture, and I don't shoot sports (at all) or portraiture (with a zoom). So I'm just saving my back and my wallet with these 'sleeper' / 'pro-lite' lenses that work like a charm.
I know it's not what the die-hard enthusiasts and pros may want, but I'm pretty geeked about this F/4 zoom (if/when it happens).