The pro's of the 150-500 OS is great reach, great IQ throughout, great stabilization system, good AF speed and great value for money.
The con's are the weight (gets very heavy when handheld all day), hunts for focus at times (more likely user problem) and it needs good light as it's f6.3 at the long end but that's about it.
The IQ is not far off the Canon 100-400 L at it's longest focal length, the AF is very accurate, which is sometimes a problem when shooting small animals in the brush or birds through branches when combined with the weight, as the slight movement will alter focus.
I've got a 100-400 L as well and recently I've been using the Sigma more often for wildlife, just for the extra 100mm reach. My idea is to use the 100-400 on my 7D and the 150-500 on my 5D2 primarily for airshows where the reach is paramount, but it's a great lens for birds and wildlife too.