I had a Nikon 600mm f/4E and had to send it to Nikon for repair of focus problems three times during the five years that I owned it. The first time they were unable to diagnose the problem, so returned it to me without a repair. The second and third times they replaced the focus motor. The warranty had expired by the third time, so that repair cost me about $600, after which I sold it. I have been interested to hear in this thread and others that my experience was not unusual. It was an excellent lens when it worked, but I didn't feel that it was reliable.