400mm f/4 DO IS opinions?

[Fast Glass]

So I finally sold my old Minolta 600mm f/6.3, I just wasn't using it like I used to and I kinda didn't want to sell it but I got into portraits for hire lately, have another coming soon right now.

But now that I sold it I thought I was gonna invest in a couple of primes or a 24-70mm. But a utterly mint 400mm f/4 DO came my way on FB market place and now I'm very tempted. I thought I was over not having a super telephoto lens, but I guess once a birder always a birder. Having IS and AF in a relatively reasonable size and weight package would be SOOOO nice compared to using a MF 600mm. Which I did with good affect.

The 400mm DO was always a lens that intrigued me and something I always thought would make a great replacement for my 600mm. Basically the very shortest and lightest modern replacement you can get. Discounting the Mark II of course. The other alternative was a 300mm f/2.8 IS, which is definitely an option. But I thought have the option of mounting a 2X extender for 800mm would be very cool.

This one alegendly has been out of the box three times since he bought it new in 2007! And it looks like it, all original box and accessories at a very reasonable price.

But I would love your guys thoughts and ideas, especially those that have owned it before.

And I'll be using it on my 1D III and 1Ds III just incase you'd like to know.