Hi Cody,
I purchased the Sigma 150-500 a bit over a year ago. It is on my Sony Alpha-350. In this configuration, the image stabilization is in the Sony camera body rather than in the Sigma lens.
For comparison, I also have a Canon 400 f5.6 with a Canon 40D body.
The Sigma 150-500 had its greatest workout when I spent a number of weeks in Peru, in everything from rainforest to 16,000 feet elevation. Ever tried to take a photo from the back of a mule, while on a narrow mountain ledge??? I managed to get some acceptable photos while bouncing in a small boat on the rain-swollen waters of the Amazon. The camera and lens were abused, including falling onto the ground on a number of occasions. And it never missed a beat.
While the Sigma is hard to hold steady from a shoulder position when extended to the 500 mm length, if you have a rifle grip (e.g. Bushhawk), it can be done. As well, when fully extended, you need reasonably good light. Nevertheless, I did get some good photos as early at 6:30 in the morning, a half-hour after sunrise (when the sun still had not penetrated the deep canyons where I was working). I found that most of the time, I use it at the 500 mm extension.
When I have done side-by-side comparison shots using my Sigma/Sony and my Canon, both on tripods, there is little appreciable difference in the images when they are cropped to yield a bird of identical size. (For those comparison shots, I photographed a Kestrel that was on a tree at Windemere Basin.) The Canon's images might be just a bit brighter, but I find that is something which can be adjusted on the computer.
In my experience, the greatest downside to the Sigma is that it does not capture and hold its focus on a bird as readily as does the Canon. I have missed a number of shots because of this. The problem is greatest when trying to get a shot of a bird in flight, but it can also be confused by foliage near the bird. I found that aircraft in flight are easier to capture than birds, perhaps because their flight path is more predictable and the Sigma is able to hold the focus better.
However, the equipment you use is determined by your budget. And, for the money, I don't think you can get a better lens than the Sigma.
I don't live in your area, or I'd be happy to lend it to you to test drive. Perhaps if we connect on an OFO outing, you would be most welcomed to try it.
Charles