As a freelance writer, people often ask my opinion of services like Elance and ODesk. And my answer is always the same.
It completely depends on whether you're looking for employment or looking to outsource?
Elance, ODesk and other professional recruitment/freelancer sites are commodity-based competitively-driven races to the lowest possible $/hr rate. As an American professional, you're not making money here outsourcing your creative talents. Because people looking for resources here are looking for the cheapest option. And the dude in the Phillipines quoting $4/hr is always going to win.
Now, on the other hand, if you're interested in outsourcing any of your professional or personal life, or have a business idea and don't want to go broke giving it a try, these services may be perfect for you.