The Canon 28-70 f3.5-4.5 is quite a noisy lens (very old AFD technology). I bought this lens because it got very good review from the website. Some say, the IQ is very near to the L lens. I found the IQ is quite good but there is purple fringing which I did not remove in the sample images below for you to get an idea. I have a 24-105 F4 L and 24-70 F4 L in the past which I have sold away. If memory serves me right and I have compared to the L lens in my archive, it looks good in terms of sharpness but as color and contrast is concern, I think L lens is slightly better. Anyway, my assumption may not be correct as those pictures are shot from my Canon 5D and Canon 60D (Color response from these cameras are slightly different). Summary, it is superb lens and for the price, it is a superb buy in my opinion. I really hope I still got my L lens with me to do a comparison. As for focus speed, there is no match with L lens. For compactness the old lens wins hands on.
Johnson's Photography 