My experience is with the Canon 500 F4.5 (non-IS) and 600 F4 IS. The 500 4.5 is very light, weighing in at 3 kg. It can be handheld (for short periods). The Northern Pintail shot in my album was shot hand held with the 500. I can carry the 500 mounted on Gitzo legs legs with a Whimberly head and a shoulder pad for a fair distance. I cannot carry the 600 for more then 50 m (I am envious of Nigel). I imagine the 500 F4 IS, weighing in at 3.8 kg, is similar to the 500 F4.5.
Another issue I have encountered with the 600 is the length. I frequently use from my truck. The added length is very noticeable when you are moving around in a vehicle. I have to be very carefull not to bang it on anything. I can move around freely with the 500.
In my opinion, the most important thing to consider is to make sure you budget for proper support. Whether you choose the 500 or 600, a gimbal head makes all the difference.
Another issue I have encountered with the 600 is the length. I frequently use from my truck. The added length is very noticeable when you are moving around in a vehicle. I have to be very carefull not to bang it on anything. I can move around freely with the 500.
In my opinion, the most important thing to consider is to make sure you budget for proper support. Whether you choose the 500 or 600, a gimbal head makes all the difference.
.. I'll keep my finger crossed wishing you a finer weather :t: ... keep us informed! 
