I just got in today the New Canon 70-300mm 4-5.6 IS USM to replace my Canon 75-300mm IS lens which I just sold last week. The differences are:
Smaller & Lighter, No zoom creep (has a Lock IF you need it), seems to focus fast - on Manual as well as automatic, has the New 3rd generation IS, seems to be really sharp. Still testing it.
So far, I like it, what bit I got to shoot with it today.
Only thing - the Canon lens hood for this sells for between $40.00 & $45.00 - I will NOT pay that kind of price for a Plastic lens hood no matter who made it, so i got a Hoya Rubber Lens Hood for $14.97