If an outsourcing site helps you get the right people to deliver good quality work, then it's best to go to such outsourcing sites but if not, then why would I care? I'd rather hire directly.
If you think it's easy to outsource because of the technology, then think again! It's never easy to outsource. You need to follow some basic steps and implement them. You can read essential tips here:
Knowing what to outsource and the right time to outsource are just two of the many things you need to consider. Also, getting the right help should be considered as they will be the one to do the job. You know it's always a risk to hire strange people who will work for you and monitor them on a distance. Finding reviews, feedback and ratings should be done to avoid hiring the wrong ones. Odesk, for one, can give you a summary of the performance of a worker, so I guess that would help.