I haven't actually got anything done through any of these sites but I have some friends who have, and I think it can be hit and miss with regard to the quality of developers on the sites.

An idea I'd like to try is a small guinea-pig project that you would use to find the better developers - get them to build it and see what the quality is like. It may cost a bit up-front but if you find good developers you could save yourself a lot of money and get a lot better results on a real project.