I use Textbroker for a number of reasons.
1. You can specify the quality level per article and pay accordingly. I wouldn't recommend going any lower than 4 stars though. I have mostly good experiences with 4 star writers, 4 star is actually quite affordable too.
2. If you don't like the article you can request it be re-written, by the same or a new writer.
3. They have US English as well as UK English writers, there's actually a .co.uk version of the site too.
4. You can create teams of writers, so if you have a large volume of requests, you can send the requests to writers you've worked with before and know what to expect from. You can also create teams for specific niches etc.