What do I think?
Since we gave a [CR3] to the EF 24-70 f/4L IS rumor, there have been a lot of questions, confusion and opinions about why Canon would make such a lens. Especially since the very good EF 24-105 f/4L IS is already in the lineup. I just wanted to give my opinion about this upcoming lens, one that I think makes a lot of sense. I'll do this in FAQ format from emails I have received over the last week or so.
This is from the opinion of a photographer, I can't touch on the video angle. As Dan Chung suggested, videographers do prefer the additional reach of the 24-105.
Will the EF 24-105 f/4L IS remain current?
I do not know if the EF 24-105 f/4L IS will remain current in the Canon lineup. My gut tells me it will slowly disappear into the sunset until it's gone, so it could last another 6 months or so. We'll have to wait and see the official word from Canon.
Why does the f/4 version get IS and the f/2.8 version does not?
I think the biggest reason it's omitted in the f/2.8 version is size/bulk. Canon's goal was to make the version II of the lens a lot lighter than the predecessor, and they accomplished that. I also don't believe EOS 5D Mark III & EOS-1D X shooters need a stabilized lens for event or portrait work. It's not genrally considered a “walk-around” lens. The f/4 version is going to be mated to a lot of EOS 6D's, and may be the only lens people have on the camera at the beginning. IS is a nice feature if you're travelling and shooting still things in low light.
There will always be some controversy on this one.
Why would I buy the EF 24-70 f/4L IS over the EF 24-105 f/4L IS, when I get 35mm more reach with the latter?
First, I'd like to mention that the new EF 24-70 f/4L IS may be longer at the 70mm end then the model suggests. Lets wait and see if it's actually 70mm, as it could be significantly longer. “24-70” has appeal from a marketing standpoint, as it's a highly desired focal range. If you look at the EF 70-200 f/4L IS, it's actually quite a bit longer at the “70mm” end.
Second, I have worked in photo retail and own a lens rental company and I can honestly say that no one ever bought or rented an 24-105 f/4L IS over a 24-70 f/2.8L because of the additional 35mm of range. People were into the 24-105 because of price, size and IS. Now compared to an f/4 version of the 24-70, would people choose the 24-105 over it for the 35mm? I think very few would given the new lenses attributes. The reduced weight, better hybrid IS system, and most likely better optics will be big selling points. The much improved minimum focusing distance of 0.2m will also be a big deal as you're getting a near macro lens as well. I also think that people will like not having overlap if they have a wonderful EF 70-200 f/4L IS, or 70-300 f/4-5.6L IS. Put a 17-40 f/4L in your kit and you have 3 lightweight lenses that would make a terrific kit for the amateur and professional that doesn't need light gathering ability.
Now, there are definitely people out there that will disagree and prefer the 105mm on the long end, I can only tell you what I have experienced. Maybe Canadians are weird? :)
Thirdly, there may even people that own both of the new 24-70s. f/2.8 for your event photography and the f/4L IS when you're out and about.
What about the new EF 35 f/2 IS?
Great, if it performs like the new EF 24 f/2.8 IS and EF 28 f/2.8 IS lenses, it will be a keeper. This lens will probably become Canon's most popular non L prime pretty quickly as it will make a nice pairing with the lightweight EOS 6D, I am really looking forward to trying one out. I also think if it's priced liked the other new non L primes, people won't mind as much due to the f/2.