1) Reach, reach, reach for birds and you can never have enough.
2) 300mm 2.8L IS with 1.4x extender is great for large birds that are stationary in a tree or nest and you are somewhat stationary with mono or tripod help and a gimbal head. I use glass taxi as permanent storage, space for extenders, flash, pro body and easy travel. Great for field sports. I will never let it go!
3) 100-400mm is great and serves a separate purpose with much more flexibility, works well in tight quarters no mono or tripod, and travels well. Everytime I think about getting rid of it I use it one last time and change my mind!
4) 600mm 4.0L IS, used and I think that I have arrived at my final destination. With extenders, gimbal head on tripod and very heavy but I knew that getting into it!
5) All used on 50D, 5DII and 1D Mark IV (purchased used)
I use them all for different purposes and bought them over time but wonder if expectations can ever be met.