I'm just catching up on this thread, but I love the photos (esp. the squirrel / heron shots) and good discussion about lenses. I have borrowed the 800, 600 I, and 400 I from CPS and really liked the reach and IS of the 800 over the other lenses but found the size (and the weight somewhat) to be the biggest inconvenience. I was all set to buy the 200-400 1.4x until I realized the size and price were much more than I was expecting.
I tend to stalk, or stalk and wait, my subjects and have been amazed that I can carry and hand-hold the 300 II all day without needing a tripod or monopod. It's definitely limiting (even with the 2x III) for small or distant birds, but the ability to get closer to them counts for a lot. It's more work, but I love the challenge of wildlife photography and my income doesn't suffer (much) if I miss shots. While I understand their use, I have no interest in gimbal heads, and even though wiser men would stay further from some of the subjects I shoot, I find the 300mm length is perfect and the f/2.8 is amazing. I agree that it's too shallow for some shots, but the AF performance and ability to defocus the background is amazing. I also like the versatility to shoot it at 300, 420, and 600mm. I've shot portraits at 300mm, and I'm covering a horse event later this week and shooting the Blue Angels in a few weeks. I think 500mm+ would be really limiting for these purposes so I'm happy with the 300.
I'm interested in the cone handle - the RRS replacement foot I have on my 300 is great for the tripod/monopod, but not too comfortable to carry around and hand hold. How did it ultimately work out?