I am mostly against lenses with image stabalization. They are slow to find focus, becase the IS unit must spin up each time, then drop into focus. This can take up to .9sec. on the Canon 100-400. that makes it slow when shooting birds or quick action sports. Also, the Sigmas are slow aperture lenses, they need a lot of light to make them shine, but when you have the light, they are fine.
I am on my second 50-500 Bigma. They are fine lenses overall. But I sold my first one because f/6.3 was just too slow for where I live, which is under almost perpetual cloud cover. I bought a second one because I now have the 7D which is good at higher ISO's, which makes up for a slow lens. Except who knew, we have had 6 weeks of dark gray days under thick rain and snow clouds, so I have not used it much since I got the second one.
Before getting one, I would make a serious study of my area. If you live in Arizona or Florida, don't hesitate though. Just don't take the advice about them being a light hog lens lightly. 