IMO, a highly underrated lens.
It provides excellent images at a reasonable price. The build quality is excellent.
It contains a unique feature whereby a macro function equivalent is provided at the full zoom position. A switch changes the mechanical linkages internal allowing ultra close-up images to be obtained. The close focus distance provides 0.5 x life size results. Being a non-flat field design, admittedly, it does not match that of a dedicated Macro lens. The end result of placing the lens in this mode, is the same as placing an extension tube between the lens and body.
I believe this feature was included to offset the poor close focus performance (at 70mm zoom position), when NOT in the macro mode. This is the known and acknowledged Achilles heel for this lens.
Under normal use circumstances, the images obtained from this lens far exceed my initial expectations. It is a excellent performer in a small package. The lens is provided with standard 77mm filter threads. IMHO it bests the original EF24-105 F4L IS performance by a considerable margin. It is lighter than, smaller than, and provides better quality images for equal or less cost, albeit at the loss of the focal length of 70 to 105mm.
Nevertheless, a really good lens.
With 3 lenses, one can have a high quality imaging set from 16mm thru 200mm all at constant f4 and all IS and a very handy macro function as well.