I've had this lens for a full season of sports now and have to say that I absolutely love it! I bought it off of ebay and it came with the collar, a UV filter on the front (112mm holy crap!), and a UV and CPL drop in filter. One of the greatest things about this drop in fliter is that it comes with one that rotates (no need for specialized CPL drop ins).
As for actual performance, it works great on both my 7D and 5DIII with very accurate AF. The AF makes a nice whir as it focuses from one place to the other, but it actually moves very fast for me. It just sounds slow due to the noise, much like an old sigma 24-70 2.8 I used to have. It works great in low light with great sharpness in the center which becomes a little soft at the edges at 2.8, especially on the full frame. By f4 it is very acceptable and by 5.6 it's awesome! This lens has performed for me very will both indoors and out, even in not-so-great lighting.
Last part? The price. It certainly cannot be neglected. Would I love a Canon 300 f2.8 L IS II? Of course! That being said, even the Canon non-IS used in terrible shape would've cost me at best 2.5x the money and a brand new Canon everything would be 10x the money. For that savings, this is one of my best bang-for-your-buck lenses and is used very often.