comparing to the Canon counterpart, I feel that they are almost on par. The AF of the L len is faster a bit which really doesn't bother mi as much since HSM is quite fast liao. The sharpness is there but find that the Sigma is warmer and the colour not as saturated as the L counterpart. Handling, both are fine on my hands with the L len slightly heavier.
But for the cost, I dare say that Sigma is worth the money spent.
$0.02