The EF 24-105mm f / 4L IS USM is a high-end universal zoom lens of the Canon L-series. It is high quality and has an excellent ring-shaped ultrasonic autofocus motor and an image stabilizer which allows shutter speeds that are longer by 3 f-stops than usual. The full aperture of the lens is the entire focal range at f / 4, which unspannend, at the telephoto end, however, is comparatively well in the wide angle range. I would recommend the lens is not necessarily for use with APS-C cameras, as the then "effective" focal length of 38 mm - 168 mm simply is not very useful (the EF-S 15-85mm f / 3.5-5.6 IS USM is an interesting alternative APS-C).
The EF 24-105mm f / 4L IS USM is a really sharp lens and its resolution is at some focal lengths even higher than the more expensive EF 24-70mm f / 2.8L USM. But distortion ("distortion") is a real problem as well as the curvature of the focal plane ("curvature of field") at this focal length (other focal lengths are not a problem) at 24 mm.
At the edge of the image may in sharp image areas fringing ("lateral chromatic aberration") occur but for the color fringes are in blurred image areas ("longitudinal chromatic aberration") not particularly remarkable. The uniform circular aperture shape creates a pleasant bokeh (blur beauty). As with all wide-angle lenses also occurs at EF 24-105mm f / 4L IS USM edge shadows ("Vignette"), which can be reduced by stopping or software correction (if it is at all bothersome).
Even if the EF 24-105mm f / 4L IS USM is not exactly small or light, but it is much more pleasant to use than the larger and heavier at 244 g EF 24-70mm f / 2.8L USM (who is not a lot of money to has available, should also be the EF 28-135mm f / 3.5-5.6 IS USM look at it).
I have the EF 24-105mm f / 4L IS USM already repeatedly enjoy working for years in operation and with the lens some photos that I find really good. It's definitely a versatile lens and I take it like having on your outdoor pursuits. When I take pictures for a client, I prefer but generally due to its better open aperture (or the concomitant smaller focus range) the EF 24-70mm f / 2.8L USM. In my opinion, both lenses are worth their money and you have to decide which criteria are most important to you himself. Of course, would be an f / 2.8 lens with image stabilization in the size and the price of the EF 24-105mm f / 4L IS USM the best of both worlds, but - so far - is Canon unfortunately forth no such objective.
A much more detailed report I published together with all the test shots, specifications and sample images on my homepage LensTests_de.