This is the Mk II version of the lens, which was introduced in 2010. Around the time it was launched, we had the opportunity to ask some Canon techies what was different from the Mk I version.
It's a bit sharper, they said. We pointed out that people hadn't exactly been complaining that the Mk I wasn't sharp. True, they agreed, but this one is a bit sharper, especally into the corners.
It delivers better slightly contrast and colour rendition, they said. The Mk I isn't bad in that department, we countered. True, they said, but this one is a bit better.
It focuses a bit faster, they said. We were pretty impressed by this because the Mk I is no slouch, and slow focussing wasn't a complaint we'd ever heard. No, they said, but this one does focus faster. And it focuses a bit closer too.
The IS system is a bit better too. It delivers about 4 stops of shutter-speed improvement before the onset of camera shake, compared with about 2-3 stops for the older lens.
All in all, then ... no single major improvement over the Mk I, but "a bit better" in pretty much every department.
So if you use one of the newer high-pixel-density bodies and you want to make big enlargements or crops; or if the IS performance is critical to you; or if you just want the absolute best image quality you'll get from a telephoto zoom - then this is the lens for you. But for the rest of us, the Mk I lens will still deliver the goods. After all, it was the workhorse of choice for professionals for a decade, and it didn't suddenly become bad overnight.
This lens can be used very well with a Canon Extender (teleconverter). The 1.4x Extender makes it a 100-280mm f/4 zoom, and the 2x Extender makes it a 140-400mm f/5.6 zoom. With either Extender, the lens will still autofocus on all EOS cameras.