I originally had and still have my 75-300 IS, but I needed something that focuses a lot faster and I also wanted sharper images so I bought the 70-200 F4L. After reading so many good reviews about this 70-300IS, I decided to sell my L to get this. 80% of me is super satisfied. 20% of me still wishes I had an L. I love the quality of the L but I do mainly drift sport shoots and the IS helps a lot. Also, the fact that this lens focuses much faster and the images are much sharper, I am really happy with this lens. I guess I'll keep this lens until I can one day afford the 70-200 f2.8 IS.
In autofocus, when zoomed in past 135 in lowlight, the lens tends to hunt for a bit which annoyed me but the results that i got from this lens compared to my old L and original 75-300IS, this lens has gotten me the most useable pictures so far. And once I retouch some of these photos in photoshop, they look just as good as the L. But since I'm lazy with photoshop, that is the reason why I miss the L a little. Anyway, for anyone really needing the panning IS mode and good pictures without burning a hole in your wallet, I highly recommend this camera.