Thoughts on AF speed (and as important - accuracy).
I think that the lens' optics are stellar. However, the AF is IMHO merely functional.
Yes, you can shoot sports/action with it and produce good images. You can also produce very good images with a manual focus lens, if you're willing to do the extra work necessary and are willing to put up with missed opportunities. This discussion (expectation) is influenced by what is currently attainable with modern equipment.
My observation with this lens was that the AF speed alone was not as fast as something like my 80-200 f2.8 AF-s. OK, fine - willing to accept that for the price of the lens! Had that been the only "niggle" I would've lived with it. The frustrating thing to me was when I would attempt to focus on a slow moving or static subject and the lens would sometimes just "miss". It was not just it's speed - it was its accuracy and hesitation that was frustrating.
I held on to mine for some months before finally selling it after I bought my 80-200 f2.8 AF-s because I really like some of the other properties of the lens - love the bokeh, minimum focus distance, weight, color rendition, etc. I really like the images here that were shot under "relaxed" conditions.
But if you shoot sports, IMHO, you may desire to move another direction.
Good luck.
