Perhaps I had a malfunctioning unit, but the lens was soft at 300mm. Way too soft for an L lens of this price. There appeared to be significant improvement at 200mm, but in a lens like this the 200-300mm performance should be good.
There is this one thing to note about the lens though - at least on a 7D, the camera *always* selects an aperture of (at least) f/7.1 when using program mode at 300mm. Even if one pushes the ISO up to 3200. 70-200 f4 and f2.8 IS:s that I've shot with always select the largest aperture (f4 or f2.8 at 200mm) in program mode. So, that would suggest that canon considers wide open performance at 300mm sub par. So no, I didn't warm up to the 70-300L, there was no "wow" factor at all for me. I was on the verge of giving a 6 for overal rating (for this unit at least), because of the price/performance ratio.