My impression of it is that basically it's a place for two kinds of employer: 1) the "I have this idea, code it up for me for equity in my 'company'" employers who either don't actually have an idea of what they want (and will needle you to flesh out their idea more fully) or have a generic "let's build the next X" (where "X" is usually "Facebook") idea; 2) the lowballer who wants to pay $500 for a job that would take a proficient team weeks or months to do.
As a "senior" level developer, I absolutely avoid these places. And I absolutely do not recommend anybody use them for work. There are far better ways to 'practice' with new languages that involve better compensation and better actual experience.