I have experience with three of the major freelancers bid sites and would like to leave my thoughts for newbie writers...It is my opinion that it is a great way to break into writing and build up clips.
However, I know many writers are not fans of these sites. While it is true that many of the jobs go to those willing to aggressively undercut their pricing, that does not mean that you and I have to stoop to undercutting our prices. I've won a lot of gigs for a fair price. Even some for more than a fair price! For example, I proofread a 317 page book for 1200$ Easy!
http://www.ifreelance.com/: while the interface is lovely and allows you to upload a good deal of "stuff," I have found that this site is the #1 place for employers looking to just "get a feel" and not really hire someone. Doesn't cost to join.
http://www.getafreelancer.com/: I have got tons of jobs with good money on this site. However, I am not a fan of its interface- you can't upload a lot of clips or photos or your logo. Doesn't cost to join unless you upgrade to a gold membership. At this site I won a gig to write 5 press releases (which are naturally short) for 150$
http://www.elance.com/: One of the most competitive. Great profile builder, you can upload a lot of clips and etc. I just started at this one so I can't say a lot. It does cost to join.
http://www.rentaghostwriter.com/: Not a lot of traffic. Difficult to use and not intuitive. I got an ongoing blog gig for $75/month on this site.
http://www.guru.com/: I don't like it or use it much.
(EDIT: Guru.com very proactively contacted me after this post to inquire as to how they could improve their service. Really, I think such great customer relations bodes well for the company. In addition, I feel I should have supported my comment a bit: I don't like Guru simply because there are not a lot of projects open to writers who have not upgraded to the paid membership. I feel the cost to upgrade is too much compared to their competitors. I get a lot of good paying jobs at sites that cost far less.)
http://www.rentacoder.com/: Didn't like it or use it much.
Please feel free to comment or add any to the above.