I've had a really great experience using FreeeUp, and I've used every type of outsource service, including Upwork, Odesk, freelancer, guru.com, etc. etc. You won't go wrong with Freeeup.
What I particularly like is you don't have to wade through hundreds of applications for random people who aren't qualified and who never read your post to begin with. Instead, you create a short job description and then Freeeup does the work of determining who is best qualified and then they send you 1 or 2 people who they think are a good fit for the job. They do the hard work so you don't have to.