A big part of my income comes from oDesk/Elance. I have just a little bit of experience with Elance, mostly I am using oDesk as both - as a freelancer and as a client.
My experience as a freelancer is great - before starting a job I am asking on every single thing about the project, which always help me to avoid to some kind of misunderstanding and also as client as me will get an idea about each other.
As a freelancer, I am satisfied on 99%. (1% is for clients who wants to build for $100 the next Facebook).
As a client, that's worse - last experience from yesterday: I had 2 designs which I needed to code into valid HTML+CSS. One is really simple, the second one a bit more complicated but definitely nothing big.
My requirement was to code the simple within 24 hours, which was client's deadline and the second one within 48 hours. I was in a long conversational with a guy from China, who was "very interested". When I finally wanted to hire him, he told me the budget is too low, the deadline so tight, then he wanted 3-5 days for coding both layouts + to increase the budget from $150 on $300.
I was disgusted, because I was waiting for that guy than he finish his dinner, then for his answers and so on. It costed me 4 hours of my time without any solution. And experiences like this I have unfortunately several, usually from people from India, Bangladesh, Pakistan, China or Philippines (no offense). How to spot them? They have usually in their titles words like "Expert, Professional, Master" and another stuff. Reading these titles drives me crazy. Who the fuck is Expert/Professional/Master in something??? If you do, you probably have not a lot of experience if you write it.
Anyway, I sit to my computer and within 3 hours I finished the first design, tonight I will work on the second one.
On Elance/oDesk are definitely scammers, but in my opinion more in between contractors than between clients.