I own this lens and the Nikon 300mm f/4. The Nikon (used with the Kenko 1.4 teleconverter) is certainly the better lens in terms of speed, build, sharpness and overall quality.
But the 150-500 still has a place in my bag. The zoom makes it versatile, especially in the woods where a bird may land right beside me or 50 yards away -- I can still get a shot either way.
The image stabilization works very well. I almost always shoot hand-held, up tall trees and so on, and that's saved quite a few shots for me, shots I would have missed with the Nikon.
But the 150-500 still has a place in my bag. The zoom makes it versatile, especially in the woods where a bird may land right beside me or 50 yards away -- I can still get a shot either way.
The image stabilization works very well. I almost always shoot hand-held, up tall trees and so on, and that's saved quite a few shots for me, shots I would have missed with the Nikon.