I don't have experience with a lens of 3.6 kG
How is it to work handheld with a lens of this weight?
The obvious answer is, it depends on your strength and what you are shooting. Last Saturday I used a 5D3, 300mm f2.8 IS and 1.4x converter all afternoon at an airshow. The combined weight is about 3.6kg and my arms got a bit tired towards the end but not as bad as I thought they might (I'm not the strongest of people). The main problem was trying to hold steading in very high winds. Certainly if I got something heavier, a monopod would be essential. The thing is, you'll never be able to judge until you try - a couple of minutes in a shop is nothing like wielding a heavy lens for hours