What are people's experiences using the 24-105mm f/4 L IS for video?
How important is IS/VR for video?
Many, many people who are shooting ther C300 like the 24-105 as their main lens.
A lot of those folks also say they would not use a lens that did not have IS.
What you are getting here is the tension between traditional "cine" usage of older, manual lenses, and more "run & gun" or documentary usage, with auto focus and even auto exposure in some cases.
It is also important to ask how much still photography you are doing, and whether you need to do both at the same time?
This is a great read on using the C300 for documentary work in Afghanastan (although he is using the 17-55 IS rather thna the 24-105):http://www.cinemaeosuser.net/index.php?/topic/58-c300-for-documentary-work/
I really like the Canon T4i for video work, with auto focus in video mode. It also has face tracking auto focus, and the ability to change focus by touching a new focus point on the touch screen.
It isn't perfect, but it moves teh T4i into more of the "camcorder" type range for event video. it also gives you new creative tools for "cine" type work. For example, I can have it track focus on a billboard that is in teh frame, then move the camera laterally and toward teh billboard while it maintains focus (so that "face tracking" isn't a perfect description for what it does.)
Or you can set up a scene with focus on one point, then "pull" focus by touching another point on teh screen. the new STM lenses (the 40 mm and the 18-135) move focus "steo by step" toward the new point silently, rather than huning like traditional USM lenses do.
Purists will chafe - "do everything manually." (I remember when auto focus first came out on cameras in the 1980's? It seemed like "real" photographers wouldn't use auto focus - like it was cheating!)
But new capabilites give you new possibilities. With every camera, you learn a "dance" to make use of it's capabilities to get the resuolts that you want." (Focus and recompose using the center focus point, etc.)
You really just need to start playing, and figure out what makes sense to you.