Generally speaking, they're lying manipulative sneaks and conmen who have no limits to the extent of the tactics they will use to part you from your money. Mental abuse is very common. Controlling the traders is a crucial part of their business model. Traders are viewed as propery, not people, and if someone destroys his life churning commission dollars for your firm, as long as you can get him to stay for two years, whatever happens to him doesn't matter. Many of the practices and behavior which are routine amongst the prop firm management would be subject to class action lawsuits in the rest of corporate America. What I am saying is not me making unfounded generalizations based on a small sample of data from negative personal experiences. It is how the industry is.
There are straight shooters out there. People who you can rely on to pay you, who don't employ manipulative controlling tactics too much (I think ALL firms want at least some control), and people who charge decent commissions. They're very, very rare, however.
This is a business of con men.
To anyone who replies "A prop firm is just a leverage providing broker, maybe you have too high expectations for what they should be doing," you are totally missing my point and out of touch with the industry.