I swithced from Canon 30D to Nikon D200, so I needed a replacement for my Canon 300mm f/4L IS. I considered the nikon 300mm f/4, but due to the missing IS/VR i wanted a f/2.8 aperture so i went for the sigma 120-300mm.
The sigma is a really great lens, excellent sharpness and build quality. The only neagtive aspect is that the "true" focal length is depending on the focusing distance. I have read some reviews on naturescapes.net and other places which have discovered the same issue (so this is not example variations).
I compared the 120-300mm with a cheap sigma 70-300mm to check the difference. At 3-4m distance, 300mm on the 120-300mm equals only 210mm on the 70-300mm . At 6-7m distance, 300mm on the 120-300mm equals approx 250mm on the 70-300mm. At 15-20mm they are about equal at 300mm.
Possible buyers should be awared of this issue. If you want a 300mm for small birds, this is probably not the lens for you (short subject distance). For sports, larger mammals/birds etc. I can highly recommend this lens!