Although I plan on getting the new version of the 100-400mm because I'd like the synchronized stabilization, I'm still debating whether it's worth the extra $$$. I have the original version and have used it ever since it was released; it's a good lens with which I've gotten some fine photos -- check this blog post:
https://forestandfield.blogspot.com/2025/03/one-more-time.html The images in that post were greatly compressed for posting.
It's not worth the extra money. The Sync-IS only benefits truly at 100mm, but its effectiveness sharply declines beyond that. At 400mm, the keeper rate at slower shutter speeds is almost identical to the MK I. If you're shooting birds in flight or using moderate shutter speeds, the IS won't offer much benefit. People are selling their MK I's everywhere right now, so the second hand MK I lenses are making it an excellen value for money lens. With telephoto zooms, lens-based IS is always superior, and most of the stabilization is primarily coming from the telephoto lens itself anyway.