I tried Demand Studios for a couple of weeks but had difficulty finding an article topic I could write on. The pay looks good at first ($15/article usually), but I found the editorial process so time consuming and cumbersome that I just could not write effectively for them. If you have a niche like writing about animals, you might be able to knock out articles for them at a rate that makes you money.
I looked at Textbroker, but there pay was a lot lower ($4 to $7 as I remember it) and the article choices were too limited for me.
I think sites like Elance or Constant Content offer better money making opportunities. I have a couple of articles about Elance which you can see if you check out my profile (plug plug).