What do you mean by "freelancing sites"? Rev share, bidding sites, job listings, content mills, marketplaces, other sites?
Assuming you mean the bidding sites (which, from your original post, I think is a safe assumption), I've tried four of the Big Five - Elance, oDesk, Freelancer and vWorker. I haven't really tried Guru since the interface doesn't work for me. It's a purely personal thing.
Note that I'm both an employer and employee on all of them except vWorker, where I'm just an employer.
Of the four I've tried, I've found Elance has the highest quality writers and editors - and consequently the best jobs. I found my book editor, cover designer and logo designers on there; only the logo designers were a bit crap but they turned out OK with some management.
oDesk is also pretty good, especially for the mid-range work. I picked up my best long-term client on there.
Freelancer, well... like AnneGeorgiou above, I'm not keen on it: it's a bit too easy to scam and spam (or was when I was using it) and I still get invites for crap-pay jobs which seem to proliferate there. Maybe just my experience. Maybe not.
vWorker is excellent for coders but I haven't tried it for writing.
Most of the others are either scams or too small to be of interest to me - they're generally a target for the low-paying employers and the freelancers who spread themselves too thinly across too many sites because they either have a bad reputation or produce crap. Given time, some of those smaller sites will grow and become more of a prospect, IMHO.
Knoji is rev share, as is Helium. WriterAccess and TextBroker are content mills like iWriter: generally, they favour the client and pay fairly low. That said, they're better than a lot of the dross work you'll find on the bidding sites and the short-term pay is a lot better than rev share (which, of course, totally outstrips everything else in the long term).
All in all, it depends what you're looking for, really. There are so many permutations that it's almost impossible to just list everything without reducing yourself to just that: a list with no other info!