I play inverted, because that's how I've always played. It's easier and feels natural, but I can understand why some people find it difficult.

I'm not sure about industry standard, but amongst my friends the norm is definitely non-inverted. Every time we play a FPS I have to switch the control scheme. This has led to several occasions where I've played "backwards" and I end up stuck between the two control schemes. It's hard to rewire your brain to control differently.