Jeff Johnson uses them for most of his outsourcing. For "dumb" things to do, like bookmarking your site at different bookmarking sites, or writing articles for you or social media. They are "cheap" to outsource and you get more done with less money/time of yourself.
Yea, I'm an mturk worker, I've done well with the site. The time I could spend making acai berry or adsense blogs, well let's just say that any time I feel I should be "researching ways to make money" I go to mturk, textbroker or find someone to work for. I'll be outsourcing tasks I know that I can make money of for sure later this year. Let's just say rather than JV with partners to share work, I'll set low prices and raise them for menial tasks that might be fun or better than writing articles for some until I find a cheap amount that Indians are willing to work for (and some U.S. citizens). Mturk only pays to India and The US, btw.