There are not many choices in the market for 500 mm lens. All but a few are prime. This lens offers the convenience of zoom lens with focal length varying from 150 to 500 mm. Its image quality is excellent from 150 to 300 mm but gradually degrades toward the 500 mm end.
HSM; OS; well built and relatively inexpensive 500mm lens
Slightly worse image quality above 400mm; OS needs about one-second to engage, not immediately effective