This new Sigma 150-600mm f/5-6.3 DG OS HSM Sports lens (see specs) is already shipping - you can check the availability at B&H, Adorama and Amazon.
Here is a detailed Sigma 150-600mm f/5-6.3 DG OS HSM Sports lens review is by Onasj.
I was fortunate to receive my Sigma 150-600 S yesterday; here are some first impressions:
1) It's a beast, but a manageable one if you are used to lugging around a 300/2.8. In fact it feels virtually identical in size and weight as my Nikon 300/2.8. It's a bit more unwieldy than the 300 prime because when the focal length isn't locked, the lens will extend from end-to-end on the weight of the glass at the end. It can be shot handheld, just as the 300/2.8 can, with some effort and practice.
2) It's very well designed. The zoom locks "hard" at 150 mm for transport, and "soft" at 180, 200, 250, 300, 400, and 600 (which also correspond to all the markings on the barrel). The soft locks are designed to prevent zoom creep from gravity, but also to be overcome by a firm twist or push/pull of the barrel. This works quite well, though I wonder if repeated use of the soft locks will eventually wear the mechanism down since it feels like the soft locks involve the lock switch sliding part-way towards lock (while the hard lock slides all the way). When shooting you can zoom the lens by pulling and pushing the barrel instead of twisting. You can assign two custom modes to the lens using the Sigma USB dock. Modes can include autofocus range (< 10 m or > 10 m), autofocus speed (slower or faster; presumably slower is more accurate). There are also two OS (aka VR) modes. The manual is poorly written and it's not clear what the difference between the two OS modes really is, though OS 1 appears to behave more like Nikon's "active" VR mode based on my tests. The tripod collar works very well and gives sharp images on a tripod even at 600 mm, VR off, 1/10 s with no obvious shake. (Sorry RRS!). It's as if actual photographers designed this lens
3) It's a pleasure to calibrate, even if it takes all day. Those of you who use software such as Reikan FoCal know that calibrating a lens that doesn't AF well, or suffers from poor optics (bad CA, etc.), is a massive headache. So it was a good sign that the calibration with FoCal on a D810 went very smoothly. Still, it takes forever to do a full calibration because there are 16 (!) AF adjustment values that you can write into the lens's firmware, representing a matrix of four focal lengths (150/250/400/600) and four distance ranges (~3 m, ~6 m, ~15 m, and infinite distance). Compared with the 35/1.4 Art in which many copies were difficult to calibrate because some distances needed back focus adjustment while other distances IN THE SAME RANGE needed front focus adjustment, my copy of the 150-600 was much better behaved, needing only relatively minor AF fine tune correction. My D810 needs at default +2 correction for a "perfect" lens, and the 150-600 mounted on this camera needed at most a +8 AF fine tune value among all distances and focal lengths tested, with an average of about +5. Which is quite good in my experience. However...