Demand Studios is a company that has hired thousands of online freelance writers who can choose from literally thousands, if not tens of thousands, of article topics to write about in exchange for pay. There seems to be a lot of argument and fighting over whether or not it is worth writing for these guys, and you can find Demand Studios reviews that say yes, no, and everything in between. However, in my personal experience, writing for Demand Studios has overall been a very good experience and the positives have definitely out weighed the negatives.
That being said, as always each person may have a different individual experience with the site, and I know many authors who were there near the beginning became fed up and left, but came back later after changes had been made and have been nothing but happy with the results since then. I've been writing with them since November, and while there have been some definite frustrations and the occasional horror story with the one editor who didn't understand the style guide and thought they were applying for a position with "The New Yorker" overall I've been very happy with the experience.
There are many pros to writing for Demand Studios. For one, there are always hundreds or thousands of assignments sitting there waiting for writers. This means that if you want to work at 2 am because you can't sleep, you can log on and find an article title you like and go earn $15 for 40 minutes of work. Another positive is that you can work around your own schedule at home, and you can actually make a full time income doing it.
Another definite advantage to writing for Demand Studios is that you get a byline with some pretty well respected websites. Demand Studios has been hired before for websites like trails.com and golflink.com. While there are downsides, these positives are great reasons many writers decide to take the jump and write for DS.