I have the Sigma 100-300mm f/4. I love the lens. I paid a bit over $500 on e-bay for it. It's sharp and focuses acurately and rapidly. I will say, however, that I wish I had something longer. 300mm just isn't quite long enough for smaller birds unless you are sitting on them. It works fine with the 1.4X TC, but that slows it to an f/5.6 and a 420mm lens without IS is very tough to handhold. I wish it had IS (VR in Nikon language, OS in Sigma language). Much slower than f/5.6 and you will find yourself only shooting in broad daylight out in the open. Serious birder photographers go for 500mm or 600mm optics (OUCH $$$). Take a look at birdphotographers dot net.