Early last year I replaced my perfectly fine, very compact Canon EF 50mm f/1.4 with the much bulkier Sigma 50 mm f/1.4. Bad move.
There has never been any argument with the speed of the AF with this lens, it's undeniably quick. When it gets a correct focus lock, you couldn't argue with the results. It's called Sharp.
Where there is argument with the Sigma 50 f/1.4 is the seemingly random focus accuracy. The percentage of keepers with this lens was so low I simply couldn't trust it on commercial jobs. Even after the almost mandatory visit to Sigma and careful AFMA, it would front focus, back focus and sometimes even nail focus.
All these issues are now comfortably in the past now that I have the three-primes-in-one zoom, the simply astonishing 24-70 f/2.8II. It outperforms my (now all sold) 24mm f/1.4II, 35mm f/1.4 and of course the Sigma.
This was the third Sigma lens that has left me burned. All were high on promise but light on delivery. Things look to be improving rapidly in Sigma land, and that's a good thing. They're coming up with great optics and once their AF issues are comprehensively solved, they'll be genuine professional level contenders. The one Sigma that keeps grabbing my attention is the unique 120-300 f/2.8. If it works properly and consistently, what very cool, flexible performance it would bring to all sorts of projects. Maybe in 2014...