Freelance programming websites

TMB writes: I am gainfully employed in a non-programming job, but am interested in doing some freelance programming on the side. I've come across several websites for hooking up freelance programmers with employers who have specific problems (elance, guru, rentacoder,...) but I would like to know people's experiences with them. Have you done freelance programming through one of these (or others)? What's been your experience?