The new lens arrived today.

Reading all the reviews I knew that the weight had shifted to the back of the lens changing the balance.
This IMO is a huge change, it is easier to hand hold than the EF 500mm F/4 II which is about the same weight.
My opinion for my use this makes it worth the upgrade.

I knew they had done away with the carrying case, but they really missed an opportunity with the new soft case.
An extra few inches to accommodate a body on the lens would have made it much more useful.

My office has the new big prime smell now, interesting odor but enjoying it while it lasts. For those that have never had one, especially new they put off an interesting smell.

Just a point is that the RF lens is the EF 600mm III with a built in adapter.
Looking at the IQ charts Bryan did the EF 600mm II was sharper than the new EF 600mm III.
Looking at the RF 600mm vs the EF 600mm II that difference isn't really noticeable, granted you have just a few MP less. Perhaps there is improvement and balance with the new R5, or the EF 600mm III when it was released had the R bodies in mind.

RF Comparison

EF 600 II vs III

EF 600 III vs RF 600

As always I worry about such things but shouldn't
Tomorrow I will be comparing the 500mm II to the RF 600mm.
I am optimistic that the new RF 600 should be stellar.