Ask YC: Real-world earnings through udemy.com, etc.?

A friend recently pointed me towards udemy.com and their online course marketplace. Up until a few years ago I was a defense contractor, and a big portion of my job was teaching very technical classes to the defense and intelligence community on the technologies we had developed for them. I always received high marks and positive feedback for these classes, and my friend thought offering a few courses on udemy or a similar site might be a good fit for me.

However, since I had never even heard of them until a couple of days ago, I was very curious what the "real world" experience is with sites like this. Of course, udemy tells a great tale about how much money you can make, etc. But is that really the case? Have any of you offered courses through a site like this? If so, how were your earnings? Did you make decent money? Was it worth the effort/time/expense? How difficult was it to stand out in the crowd? I notice a lot of classes in the areas I'd offer courses -- information security, Java development, and especially iOS development. Is it hard to stand out in a marketplace like this?

I'd really love to hear about your experiences!