I am the worlds biggest doubter of new is better than old. I am also as tight as a camels backside in a sandstorm when it comes to buying a new lens. The price of the old MKI IS is at its height so i got a good offer and sold it. Had it 3 years never lost a penny. Anyway, bought the new one used but mint. Well, it looks the same, is the same colour and has Canon painted on it. Thats the limit of the similarities. I can honestly say im totally amazed at it. The IS is 4 stops. I just cant get used to shooting at 200mm and getting keepers at slow shutter speeds, hand held. It worked in a rain storm when the camera, a 1D4 packed in but above all like the other guys said, the colours and tones are amazing, crisp and accurate. Its almost, and im splitting hairs here, almost as sharp as makes no diferance to my 85mm prime. Its autofocus on a pro body is laser fast. Is it sharper than a mk 1 No when stopped down but wide open, yes and you can tell which is more important.
Mate, its worth every penny. It really is that good. But its no better than the 70-300 IS L which is smaller, lighter, longer and just as well built but looses a couple of stops of light. And you cant use extenders.