has a potential
Through the course of the time i have used the canon 70-300 do all i can say is that it needs improvements. When i was in the perfect conditions where it was 6:00 and the sun was behind my back this lens is delightfully tack sharp. The canon 70-300 do suffers from inaccurate auto focus. it seems that the lens focuses behind everything just a little. not enough to ruin the lens completely, but enough to not be able to see the minute patterns in an eye. The lens has almost no chromatic aberrations due to the ingenious diffractive optics.
Another thing to note is the REALLY heavy zoom ring, this thing is hard to turn. the lens also has lens creep meaning that the end extends like zooming when facing the ground. but luckily the lens has a lock preventing it from creeping in carry.
In the end the entire purpose of this lens is to have a smaller and lighter telephoto zoom, and canon did just that. of course the mark ii version that will never come out would probably be awesome.