Elance - very good for getting new and repeat business. We get most of our income through this site. As expected, job awards tend to come in clusters so there is some downtime. We use this to work on our own website and put in bids on other freelance sites such as:
iFreelance - no luck here so far. I think this will end up being a waste of money. There are very few assignments that are even worth bidding on (maybe 2-3 projects a week). We haven't been awarded anything so far. This site doesn't charge commissions, so that is good. At this point I just hope to land 1 job to get back my 6 month membership payment.
Guru - just started bidding on this site on June 1st. No awards so far but there are a bunch of good projects and buyers. You get 100 "bids" a month, so I am burning through them. I asked the Guru helpdesk if I could link to my Elance profile to show my feedback and they said yes! That is good since I won't have to start from scratch building a reputation there.
Demand Studios - started turning in work on Monday and just got paid for my first article. These are simple to write but tracking down images takes a little time. The format for entering the articles on the site appears to have a couple of bugs (the bullet lists don't show up correctly), but nothing major. Just follow the Writer's Guide closely and you will be fine writing for this site.