Thanks everyone for commenting. I've "convinced" myself that I "need" to get a 400mm f/4L IS for myself and so will be buying the 200-400mm f/4L IS later this March. I've given up my earlier plans (for now) for the 300 f/2.8L II and the 600 f/4L II in favour of the zoom. Versatility first and the speciality lens can follow later.
I'd consider the DO lens but am just not convinced with it given that I cannot even try one in my country (nobody I know has this lens and it hopelessly out of stock with the Canon stores here).
Cheers ... J.R.
That's a real shame you can't try one, I don't know why Canon don't push it more!!
Well I'm sure you'll love the 200-560 f4-5.6 (which is what it really is...!!) but I hope you aren't planning to carry it a long way!!
Don't know where you are geographically, I've lent mine to a few guys in Scotland and they've al bought them!!