Since birds are my primary interest, I bought a 600L/f4 IS several years ago, and would do it again today. Whether the 500L/f4 IS would be a better choice (cheaper, smaller, lighter) obviously will depend on your subject matter. I find that for birds, even with the 600L, I'm using either a 1.4x or 2x teleconverter a lot of the time, and on occasion, even both.
I carry mine in a Kinesis long lens case with shoulder harness (and put the 1DMk2 body in the pouch on the back), and it fits well into most airline overhead bins (even on some smaller jets). It will often get some comments about size (or cost) when going through the x-ray check-in process, and I'll occasionally need to take it out for them to physically examine, but so far it hasn't been a problem in travels to Costa Rica, Canada, New Zealand, various locations within the U.S., etc.).
I'm in the minority in not using the full Wimberley head - I use the ArcaSwiss B1 and Wimberley Sidekick (mounted on a Gitzo 1325), and it works well for me.
Hope this helps.