I'm interested in this lens as an alternative to the Nikon 500 f/4. It's significantly less expensive, smaller and ligher than the Nikon, has better minimum focus distance, but at the cost of 1/3 stop. HSM is supposedly almost as good as AFS. Anyone got some real life experience with this guy? Used copies seem hard to come by so I've picked up a used 300 f/4 AFS that I can use with my TC's for the short term, but I think I'll need the longer length as I've got an itch to go bird shooting.