The Sigma 50-500 OS is a nice update of its predecessor; it has kept good image quality (impressive for a 10x zoom!) and it adds the very useful image stabilization, even though it is not very effective: it helps, but it does not come close to the impressive 4 stop IS used on recent Canon and Nikon lenses.
The 50-500 OS has only one big problem: price. The old 50-500 was around $1000, the new 50-500 OS costs $1600, a price that put this lens in direct competition with the Canon 100-400 IS. So, if you had to choose between Canon 100-400 and Sigma 50-500 OS at the same price, which one would you buy? I think that the majority of people, me included, would go for the Canon. There is not an huge difference, but there are many little things that overall makes the Canon a better lens: slightly better image quality at the long end, much faster push-pull zooming, slightly faster AF, more effective image stabilization, better built quality, lighter weight and smaller size.
In conclusion, the 50-500 OS is a good lens, but I would not buy it until the price drops to more reasonable levels, as its $1000 predecessor.