This lens is a great general purpose lens. If I was unsure of the conditions of where I might be going, or if I were just going someplace new for vacation, this lens would be the first one to go in my bag.
24-105mm is a great focal length to have. 24mm is a fairly wide-angle on a full-frame camera, while on a crop sensor camera, it is only a moderately wide-angle. This is a zoom lens and it zooms to 105mm which is a long focal length, and you’re able to shoot from farther away.
The great thing about this lens is that it has a constant aperture of f/4. Although not excitingly fast like an f/2.8 or even an f/1.4, a constant aperture helps you maintain exposure and DOF through the focal lengths, unlike cheaper zoom lenses.
This is an L lens; L stands for Luxury, referring to the Canon’s best line of lenses. This lens features a Ultra Sonic Motor (USM), which means it focuses really quietly which can be really useful in certain situations.
This lens also features IS, Canon’s image stabilization technology. Usually, you are supposed to have a shutter speed of at least the reciprocal focal length to prevent camera shake. With 100mm, you should have at least a 1/100th of a second as your shutter speed.
However with IS, 3 stops of being able to be handheld is provided. 3 stops is equal to 8 times the amount of light! With 100mm, instead of 1/100th of second, you can use 1/15th of a second to prevent camera shake. This makes a significant difference, especially in low-light situations!
In my opinion, the most beneficial aspect of this lens is that it is also fully weather sealed. So worry not when you are out shooting with this lens. As long as you don’t drop it into a pool of water, it should do fine with the abuse of rain, hail, sleet, snow, or whatever weather ailment that is thrown at it.
(of course, to take advantage of the weather-sealed qualities of this lens, one must first have a filter in front of the lens to complete the seal; along with a weather sealed camera, as well as a 580EX flash to protect the hot shoe component on the camera).
The only gripe that I have with this lens is that it is only offered at f/4. It would definitely be better if it was a faster lens. There is a f/2.8 counterpart: the 24-70mm f/2.8L. However, that lens is offered without IS.