The Canon 24-105 F4 L is a pretty common lens. Canon had a bundle deal with the C100 and lots of others have picked this lens up along the way from their DSLR gear or other deals. So while I've worked with the lens before, this was my first time operating with it for the better part of the day rather than it being on some one else's rig.
First, yes its optical performance from wide to maybe 70mm is good, even wide open.
Next this is the first time I've had an IS lens and operating without a rig. Usually I've run into this lens with proper camera support that runs the gamut of gear. It was also me hand holding this lens. Now in the past I've used my rokinon 14mm cine on a 60D plus LCD VF with great success. Never had a problem keeping that setup steady and it was critical for making a lot of B roll on a project last year when the big camera couldn't work. I also work with the OM 50mm often in similar config for shooting close ups like in my soon to be released short film. That said, I'd never go past 50mm. Today I'm out shooting at 80-105mm, IS on, and getting really good shots. Shots I have no right to be getting even though I'm pretty steady to begin with. Oh I hear the I told you so's starting already. Honestly I've never had a need to shoot hand held like this before and I wasn't using a full size camera. I've always had some sort of camera support or lens combo that I could work with. So here I am shooting panning shots of a car driving down a country road at 105mm and, well, it looks _almost_ as good as if it had been shot on a tripod...or maybe a tripod that wasn't the best. There are points where you can see the IS sort of max out and then catch up, but by and large if I had tried this shot w/o IS it simply would not of been usable hand held. So I'm something of a convert now to IS for SOME shooting situations. I'm considering adding an IS / AF lens to my collection with the upcoming C100 continous focus upgrade simply for reality / doc shooting. For my normal type of work, this really isn't something I'd ever worry about.
Thats all the good about the 24-105. Now what I find unacceptable. The lens goes seriously dark at the 105 long end. Its not a 1/2 stop, its more like 1 to 2 stops dark. Marked as an L lens with constant F through out the zoom range this is a real misrepresentation. No surprised there are so many of them up on ebay. Its a good starter lens, but no one deserving an L designation. Ok, its got some rubber rings on it for weather sealing. That's great, that's valuable for some shooters, but its not a critical feature for everyone. If I can come into it for $500 on a canon deal I'd consider possibly keeping it but I think there are other options. One option is the canon 17-55 F2.8. Much better match for typical shooting on a S35 size sensor, an extra stop of light never hurts and makes for better bokeh.
The AF on this lens also tended to hunt around. I was never thrilled with my now gone Tamron 17-50 2.8 ( see the review ) and how it was slow for AF. Perhaps the AF upgrade to the C100 will make the lens better, but in simple shooting condition - head and shoulders shot, outside in overcast light, distant simple sky and field background the lens hunted around. I manual focused every shot because it was way faster. Sure that F4 doesn't help but I had a high enough ISO in daylight this should not of been an issue.
There you go. A lens with some real pluses and minuses. If the pluses like weather sealing are critical to you, the focal length range works for what you do, you shoot a lot of bare bones hand held and the slower F4 doesn't bother you this might be an ok lens. Me, I'm gonna try the Canon 17-55. Anyone care to say how their experience with this lens is ?