If you're not aware of how these job sites work, let me break it down for you. Writers (or other types of freelancers, such as designers, administrative professionals, etc) will post a profile on these sites, listing their experience, background and how much per hour you prefer to earn. You receive a certain amount of "connections" per month for free; the number varies per website. You use these connections to submit proposals for jobs that you select from a job board. If you want additional connections you must pay a fee (starting at $10 per month and up, depending on the site) and you can receive as many as you're willing to pay for. Most jobs require one to three connections depending on how much the pay for the project is. The higher the pay, the more connections required. Sometimes clients will invite you to submit a proposal; when this happens, you aren't charged any of your connections for your submission.
There are legitimately good opportunities available on some of these sites; you just have to learn how to read between the lines. For the most part however the projects available on these sites are extremely low paying. You will see entries such as "50 articles for $20" or even worse. These project posters are not looking for real writing; they are looking for anyone with a computer who can string a few words together and stuff them with keywords. Real honest high quality clients understand and appreciate the job that we do. They are willing to pay accordingly for quality writing.
The other issue that I have with content mills is that, as long as there are writers who are willing to write for pennies, the more clients will assume that is the norm and prices will stay low. This affects all freelancers. If you are a hobby writer -- someone doing this for fun and a few extra bucks -- then by all means, accept that kind of work if you so choose. But if you are truly trying to market yourself as a business, steer clear of content mills. Your bottom line will thank you!