Newest 600mm lens
I have this lens and it is a stellar performer. But the main reason why I bought it was the weight savings. It is now about the same weight as my old 500mm f4 IS. And like some of the folks here, I sometimes handhold it for birds in flight.
Not to put a damper on this fine lens, I recently had an issue with this lens. The IS module would jump like a jack rabbit inside the lens and it would rattle loud enough for anyone around to hear it. The problem was that it occurred randomly. On 3 overseas trip, it happened to me after several days of shooting. I had it brought to Canon service centre 4 times. The technical people were scratching their heads as to the cause of the problem. Eventually, Canon changed the IS module and the circuit board. They refreshed the firmwares on the lens and my 1DX. Now everything seems to be working just fine - I hope.
Despite the problem I faced, I would still recommend this lens to anyone.
I was shooting with a guy from China that had this lens. He has had the IS replaced and it still did what you are explaining. He was very annoyed. He was using the 1DX and III extenders on a tripod without IS. he said it would start jumping or something so he left IS off.
I wish I would of gotten his e-mail address.