An excellent compact zoom
This diffractive optics zoom lens packs a real punch, although despite the other comments posted it isn't really a featherweight, being heavier than some other comparable focal length zooms. But what it lacks in lightness it makes up for in compactness and performance, making it ideal for travelling. It is also ideal for handheld work, with the weight and compactness aiding stability, backed up by the excellent two-setting stabilisation system, which is very smooth and quiet.
it isn't a particularly "fast" lens having limited maximum aperture of f/4.5 at 70mm and f/5.6 at 300mm. But with modern DSLRs having high sensitivity this is less of a problem than it would have been with film. Furthermore, I have tested the lens across the aperture range and I find the sharpness to be comparable at every setting, which means that unlike with some other lenses that need to be stopped down to f/8 or even f/11 to achieve optimum sharpness, this lens delivers acceptable performance wide open. The fact that this lens delivers such good performance wide open and is so compact makes it ideal for sports photography without the need for a tripod or monopod, for example.
This comparison can be seen in the example photos I attach. The first three photos are of a building and trees around 750 metres distant, at 300mm, f/5.6, f/11 and f/22. The final two images are enlargements of the tree to the left of the building, at f/5.6 and at f/22. The image at f/5.6 is marginally sharper, probably due to movement of the branches (or the tripod!) during the longer exposure at f/22.
The sharpness and contrast are probably not quite as good as could be achieved with a fixed focal length telephoto, but that is the case with just about every zoom lens and in the majority of practical cases, the difference will not be particularly noticeable.
Overall, I rate this lens as excellent. In terms of value for money it is not quite as good, as the price of this lens new is hundreds of pounds higher than a comparable "L" series zoom. But good examples can be found on the second-hand market, which brings this lens within the reach of amateur photographers like me. (But since this lens is so portable, look out for damage due to the lens being knocked about in use).
NB: the test images were taken with an EOS 100D, so they are not "full frame" images. This lens is capable of being used with full-frame cameras, however.