The Sigma 120-300mm f/2.8 EX DG OS HSM is a cracking piece of kit. At the apertures it's most likely to be used at it's very sharp, autofocuses quickly and offers the kind of sterling build quality you need from a lens that's likely to spend most of its working life outside.
The telephoto lens is made from durable, sturdy metal and has decent weatherproofing for outdoor shooting. Image quality is also excellent, particularly at wider apertures and using the built-in impressive optical stabilisation.
The waterproofing falls down in the lens's lack of a rubber seal on the lens mount, and parts of the lens are tricky to manouvre at times.
The big maximum aperture undoubtedly makes it a more unwieldy lens, which isn't particularly welcome news, but that will often make the difference between a comfortable shutter speed and one that produces wobbly shots. The Sigma 120-300mm f/2.8 EX DG OS HSM is well worth a look.