As a dyed-in-the-wool Canon owner I have stayed with their glass up to now. My Canon 28/135 IS, just wasn't getting the job done. My 100/400 was too much for the positions available.
I shoot a lot of Horse shows, especially Dressage events. Once the events starts, you aren't allowed to move about (distracts the horse and/rider).
The far side of the ring gave me less than full frame Horse and rider and the near side filled it too much.
The Tokina really takes care of both situations very nicely. I have found that it stays very sharp at both ends. Autofocus is quick and accurate. The only problem I had with AF was an attempt to Macro a bee on a flower and I didn't back off enough at first, the motor tried but I exceeded the closeness to the subject..Color is good. The Parafocal feature is worth the price by itself! No tendancy to creep when fully extended. Strongly recommend this lens for anyone that needs a quality, durable lens.