my experience is different.
I am using odesk for the last 4 years spent there tens of thousands of dollars, most of the time it is incredibly useful. granted i had some failures but with "normal" hiring I had higher failure rate and each failure is much more expensive.
the biggest power of odesk is the speed of availability. within minutes i get many qualified contractors. ready to work. speed unmatched.
last week i ordered a logo from a contractor on odesk and 24 hours later costing us 44 dollars, i got a logo for my new startup. (there was a full process of selection) this logo would cost me hundreds of dollars going the normal path and will take forever. (you can see the logo at www.glue.im)
a while ago i got in trouble while administrating a server, 15 minutes later I had a great guy, a knowledgeable system admin to the rescue - problem solved. he also showed me what i did wrong $12. now i use him to save my time.
as you can see English is not my native language, 3 hours before deadline for ycombinator submission, I need a quick English fixer 30 minutes later Evan from florida is with me on skype going over the document. $50 incredible job for the time he had.
ramen profitability - at glue (http://glue.im) we try to keep burn rate low while supercharging our code writing. with Odesk and our charm, we keep burn rate < 5k while having 3 full time employees writing code + 1.5 founders. (the other half founder is writing this) we develop our mobile app on Android, iOS, and Tornado backend together. with a diverse team from England, Ukraine and Bulgaria.