I've used GetAFreelancer (GAF) and RentACoder (RAC) and I have to say that RentACoder is winning the battle. I've had many jobs at RentACoder and only a few at GetAFreelancer but more than that, it seems that as with anything else, there are more than a few people using GAF as an advertising site.
I applied for one article writing job only to get a quick reply back from the buyer. I was excited as I opened the email only to be sent to a website. Some buyers will give a website link to show the site they want an article for, etc but this was just plain advertising. If I paid "them" they would give me a good rating in a few different freelance sites. I have yet to find any spam, advertising in RentACoder.
I also wonder how serious the potential buyers are at GAF because sometimes the auctions are a month long and even months long as in 2, 3 even 6 months long. If you want to buy articles or programs and other coding why on earth do you want it there so long?
Many of the jobs I seen the pattern and realize that many "buyers" are just trying to advertise their websites and use GAF, plus in any given auction that looks really good such as its something that you can do, data entry, article writing, research, etc, and it pays good, it is hard to get. There can be HUNDREDS of other bidders.
I really like RentACoder but it has its faults as well. Well it has A fault. That fault is the navigation. I found it hard to navigate the site and needed the walkthrough of a friend who was using it as well. In comparison GAF is easy to navigate and has a clean interface, RAC does not.
If your looking for a freelancing site, I suggest you try both and stick with the one that works but if you like both then you should use both. Don't forget that RAC and GAF are only two of the many online freelance sites. Try multiple sites to find the one that suits you. I'll stick with RAC but I'm also signing up and trying other sites. I'll let you know what I think of them.