Well, it's been about a month since I purchased the Canon 24-105mm 1:4L lens, and I thought I'd take a moment to give a brief overview of experiences I've had with it.
Although I am still work with a crop-sensor body camera (I see a full-sensor investment in the near future), I am very impressed with the quality and range of the lens. As always, I recommend prime lenses over zoom lenses, but it you are working in any kind of photography requiring quick adjustments or are on a tight budget but want a top-of-the-line lens, then I would highly recommend this model.
The first thing you will notice with this lens is the weight. Though it is a hefty lens, it greatly makes up for that in other departments. Strictly focusing on the build, the lens includes very smooth adjustment rings for both the focus and the zoom. Stopping is also a breeze, and doesn't involve any kind of jerky motions (for those concerned with using it for film/video).
Picture quality is spectacular. Even on cheaper crop-sensor bodies, the lens performance is astounding. The photos are crisp and clear. The range in depth of field is something to drool over. Overall, it is very professional-grade. As Canon suggests, this line of lenses is built from the highest quality of materials and geared towards more advanced users. This model of lens includes both image stabilizer technology and an ultra sonic motor, meaning quick (and quieter) reactions with the auto-focus and more controlled image quality. It's really the perfect lens for outdoor/sports photography. Not only does it have a very versatile range of focal lengths (the most common ones for any introductory to intermediate photographer), lens settings can be adjusted quickly, when shooting on the fly.
The only downside that I have noticed so far is the limited aperture/iris size. Completely open, the aperture can only go as far as f/4. While this hasn't been proven to be too problematic, the lens could potentially fail to perform in low light situation. Images begin to show obvious signs of grain, as the ISO strains to compensate for the aperture.
Though I don't have any field tests ready to go (this was more of a quick overview than an actual review), I have used the lens in a few recent projects. Personally, I've been really impressed with the results.
If I do any tests with the lens in the near future, I'll make sure to post them on here. Until next time!