I replaced my older 500mm f4 with the 800mm f5.6 in 2012. This opened up new avenues for me in bird photography, the extra reach is phenomenally useful. It doesn't weigh a huge amount, but at some point that year I strained my side and it hasn't ever got better. Was it to do with carrying the lens? I don't know - it could have been on Shetland that autumn, where I carried it all day every day. Anyhow, carrying the 800mm now, even though only 700g heavier than my old 500mm, seems now to make an adverse difference to me. If I'm working close to the car, and I know the bird is a small one, the 800mm comes every time - for instance the Red-backed Shrike at Canvey earlier this year (and, I wish, for the Nottinghamshire Pied Wheatear but I didn't even think that would be photographable!). But I'm not sure I'd like to carry it around with me all day long any longer - I must be getting old! However birds in this country tend not be particularly approachable, and for that reason I choose to keep it, as it gives me a huge advantage, but it's not a lens that I actively carry around just on the off-chance that a photographic opportunity might arise. In other words it has very specific uses, but is not what you might call versatile. And it's huge!. Many airlines in Europe have prohibitively small hand luggage allowances, and as you know I have started to travel quite a bit. I realised that with my dodgy side and increased travel that selling my old 500mm might have been a mistake. Such is life, you don't know what you miss until it's too late. There was only one thing for it - start saving to buy it back!
In the meantime of course, Canon released a new 500mm, the Mark II. It weighs 600g less than the old version (18%), focuses closer, and has a more effective IS system. The older lens was no slouch in the quality department, and this new version is at least its equal, and the fact that I had to tone down my Photoshop sharpening points to the fact that it is even better. The only trouble is that it was nearly double the price of the older second hand model. I agonised for a while, and then took the plunge - you only live once. And I am glad I did, the new 500mm F4 IS Mark II is an incredible bit of kit and I get stacks and stacks of use out of it.
Having now used it extensively for about a year I can't think of a lens better than this one for general birding, or travel where birds might be on the menu. For starters it packs into a tiny bag, unlike the 600mm and 800mm. The value of this cannot be underestimated, it is so easy to pack. I can take this lens anywhere, even on the smallest of planes, nobody ever questions the small bag, which also contains a 1d body alongside (i.e. not mounted), both converters, a standard sized lens like a 100mm macro or 16-35mm zoom, and various small items like batteries and cards. Monopod strapped to the side, I have a package that can go anywhere for bird photography and be perfect for most birding situations. Partnered with a 1d body it weighs in at less than 5kg. I can handhold it all day long, and with my unique horizontal carrying system I barely notice it on my shoulder if I'm out birding, and it seems not to aggravate my side too much. I've been doing a fair bit of travelling this year, and this lens has been everywhere with me. Without even a monopod, I'm finding that it delivers great image quality time after time. And it takes the 1.4x converter without blinking. I swear I can't tell if I used it or not. On my recent holiday to St. Lucia, I used it handheld 90% of the time, often with the converter, and I'm perfectly happy with the results. It takes the 2x pretty well too, and just this past weekend I used it with both converters stacked around a 12mm extension tube with perfectly acceptable, if not spectacular, results. The cost is the only downside, especially when a mint example of the older model can be had for around 60% of the price at the time of writing, or a saving of over £3000 - this amount could take you a long way!
Below, a female Antillean Crested Hummingbird, taken handheld at 700mm (f7.1, 1/2000s, ISO 1000)