I have one and the 600 is a great lens. My previous was a 300 2.8L with 1 .4 extender and while good it still did not have the reach or sharpness this one gives. Add a 1.4 extender and you have 840mm and that's before any chip crop factor multiplier. Just like a new macro lens, you will find a new world to explore with it and if you choose to keep it will still serve you well into retirement....though you might need to hire a caddie to help you with it.
Oh man, you guys aren't helping my bank account.
Seriously? Aggggggggg, indecision ....