I sold my f/2 135L and my 70-200 2.8L IS (I) to buy this lens. Absolutely no regrets. The 135 was an amazing lens, but without IS, I was just not always willing to shoot with it under about 1/100 due to camera shake. Also, I found the f/2 a little tough to manage in my portrait work, so the 2.8 is good enough for me.
And the sharpness on this lens is nothing short of spectacular wide open... If my version I of this lens was a '7' that would make this one a strong strong '9'. It is that much better.
The older I get the more I appreciate it when I spend the money on the best available. Don't go cheap - just sell off anything you have in this range and just get it! :-)