Been using Odesk for years. They have a great employee monitoring tools and logging systems. Very easy to keep track of and organize your staff via their propitiatory web-based tools.
odesk is clearly the most advanced. GetAFreelancer used to be ok, kinda gone down the tubes with ScriptLance recently. Don't get me wrong they're pretty decent sites to find work on, you just need to be a very active member and showcase your works to really prove you have the skills. eLance is a joke, completely gamed by larger corporations there's no way a freelancer could make money there.
odesk is what i use and have been using for the last 2 years now. I have used guru and getafreelancer but odesk is still best.
odesk if great. has a workroom with a great little feature that lets you "spy" on your workers to make sure that they are working!
I personally believe the best freelance site to use is Elance.com. On here you can norrow down your searches by the zip code and its pretty cool. The prices for labor are cheap as well. Hope this helps by the way. Steve P.
I see a lot of different answers here. The question is are you LOOKING FOR WORK or PROVIDING WORK i think if everyone who posted were to add which side they are on, it will help make more sense as to their responses.
I've used elance and guru in the past without any problems. Never tried odesk but the comments about scriptlance and getafreelancer are true about it going down hill.