I owned a f4 Canon 70-200 and found it a little lacklustre - it certainl;y didn't wow me with its Iq at any focal length, but at least i got what i paid for it in part ex against a sigma lens.
I've used the Sigma f2.8 and its a good lens - not far if anything behind what i would expect from the Canon, but maybe my view of the 70-200 is blotted by a bad copy ?