You can probably tell I'm a big fan of more than a few lenses that most brush off as also-rans or worse. The Fuji 18-55 f/2.8-4 kit lens, the Canon 17-40 f/4 L, the Fuji 18mm f/2, and obviously the one in the title. There are others that I adore that aren't on anyone's list of super-sexy glass but I wanted to discuss a few things specific to the Canon 24-105 f/4 L. To be even more specific I'm referring to Version I of that lens as I prefer it overall to Version II. Why's that you ask? Two things… Size, weight, and price.
I've mentioned a dozen or more times that the scenarios I find myself making pictures of two factors influence my acceptance of the above mentioned lenses. With how close I work and typical setups the corner performance of any lens never comes into play. Either due to crop (4x5 or whatever), and even if not cropped the corners are never ever never in perfect focus. The above is the 24-105 L at 105 wide-open. Arguable this is where that venerable workhorse zoom is at it's very worst overall.
Never mind the above. Instead let's just talk about real-world results as well as technical testing of the above 24-105 f/4L version 1 vs some other lenses in terms of performance, value, and convenience. The Canon can be had for next to nothing. The price is somewhere in the ballpark of $400-$600 for a working, good looking, and hopefully well performing sample. I've had two of them and both proved to be "good samples". I'll allow for the possibility that there are "bad samples" out there since I've heard such huge diversity of performance reports but I've never used one that I didn't consider okay. I suspect many of the poor performance reports are probably not due to actual lens performance.
Without a doubt everyone knows that the 24-105 is not up to the performance of the 24-70 f2.8L version 2. I personally don't care because I really don't like that particular focal range in a zoom. I end up swapping lenses or bodies as much as if I were using two primes in many circumstances. I'd much rather give up the 24mm end and take a 28-85mm (hence why I like the Fuji kit lens). Truth is I'd rather the 24-105mm be a 28-105 or even 28-100 and either be a bit faster and/or a bit smaller lighter. I don't care about the 24 end. I do care about the 28mm though, and the 50mm, and the 85mm, and even the 100/105mm.
Oh the above photo is not the 24-105, it's actually the 100mm f/2.8 Macro also at f/4. At any remotely reasonable reproduction sizes in both digital/web or even print I will assure you you'd be hard pressed to tell the difference. Even under close inspection most would pick the "better lens" entirely based on my random choice of focal point and what that plane of focus happened to render sharp. Why would I include this ringer? Well I can assure you that the 100mm L series macro will blow the shit out of either zoom (and many other lenses as well) in terms of resolution, especially stopped down one stop. Who cares.
The real question is how does the old Canon 24-105 hold up to some other competition? Competition as in say Fuji or Sony which are all the rage in "switching" now. I've already did a brief comparison of the Fuji kit lens to the Canon that hopefully showed that the Canon combo I use handily blows away the Fuji combo. Of course that's pointless due to the Fuji's diminutive size and other positive points. A more pertinent comparison is probably comparing it to the Sony offerings.
Early on it was easy, the only real competition was probably the Sony/Zeiss 24-70 f/4. That comparison was easy. Size wise the Sony lens/body combo sorta wins if you are looking for a little bit smaller. On the other hand the IQ of the Canon wins since that lens really blows (where I typically would use it). Versatility goes to the Canon as well, much better range for me. Value, you have to be kidding, Canon of course given the price of a version 1 24-105.
Now it's a bit more difficult as on the Sony side you have a relatively new "G" series 24-105 f/4. That's a very direct replacement on an A7 or A9 body. Reports are pretty positive about that newer Sony lens. Here's the rub — The Sony is more than twice the cost of a version 1 Canon but wait, there's more… There the same size for all practical purposes and wait for it… Based on some very technical testing of multiple copies it's a toss-up in terms of IQ. Yep, no shit. The Sony is conclusively better at 24mm, probably better at 105mm but things get really murky in the mid-ranges of the range. In many cases the really old Canon actually betters the new Sony. For my own uses that mid-range is more important to me, for you it might be different.
Photos made with the Canon 24-105mm f/4 L and Canon 100mm f/2.8 Macro L. Lighting was Profoto B2 with Profoto umbrella deep silver L and diffuser. Two quick photos I made never intending to compare the 24-105 and the 100 macro. It actually was a quick and dirty setup to demonstrate how close the results are between the Profoto deep umbrellas and a large octabox. I just happened to switch to the 100mm Macro to get a wee bit closer for a few shots and ended up making two nearly identical frames. Sorry, not super close due to quick and dirty hand-held setup.