Its AF (in the last two models) hasn’t been able to keep up with a Canon or Nikon prime, and it doesn’t sound like Jason has ever shot sports with one (either the 300 or 400′s used for baseball/football/nearly everything else).

That’s the reason you’ll rarely see a pro with a 120-300, unless they just couldn’t afford $5.8k for the ‘big two’ branded lenses.

Also, Sigma used to sell these lenses for $2k, before all the prices went up a few years ago, (while the smarter people are selling theirs for what they bought it for) although compare that to about $1600 for nikon 70-200 VRI-II to $2400 over the same period, even though I honestly doubt sigma needed the same 50% markup for overhead.

If you need to nail focus every time, in any situation, and take a tremendous beating while at it- Nikon/Canon 2.8 primes or 200-400mm zoom.

If you need to save 2.7k, (and don’t mind lower build quality, and an imperfect focusing system) buy the current gen 2 sigma, or wait for the new one. 

They also say it shares the quality of a 300 f/2.8 prime, but I honestly doubt it would look as good as the prime with 1.7x and 2x tcs. The prime is still smokin, even at 2x.

Of course, being a discount brand, they have a lot more QC issues from lens to lens, at least comparing it to my Nikon lenses.