Quick update: Sigma's newly announced 150-600mm f/5-6.3 DG OS HSM Sports Lens (unveiled last week) already has a confirmed price point: $1,999. It will be available some time in October, with a specific date not yet given. At that price point, it's much higher than the Tamron we reviewed earlier this year, so Sigma must believe the performance and build quality to outperform the Tamron.
You can pre-order the Sigma here. No pricing or availability was revealed for the Contemporary version of the lens at this time.
When I saw the price on B&H late yesterday it caused me to immediately purchase the Tamron. I really would love to give the Sigma Sport a try (like all the recent Sigmas, it's a great-looking lens), but upon looking through the Tamron 150-600 pool on Flickr, it appears to produce very sharp, good-looking images. I can't imagine that at the same apertures and focal lengths that the Sigma Sport will be significantly better, at least not enough to justify double the price. But I could be wrong.
If this end's up working as well as the Canon 400mm 5.6, I might have to pick one up.
For the majority of sports I have shoot this lens is to slow and to expensive. I would shoot my 300mm f/2.8 with an 1.4x adapter for a 420mm f/4 or with a 2x adapter for 600mm f/5.6.
It will be interesting to see the MTF curves and focus acquisition speed on both new Sigma 150-600's. Here's a recent photo with the Tamron 150-600mm VC - http://daveblinder.wordpress.com/2014/09/18/bird-photography-in-cape-may...