I have tried a bunch of them over the past 12 months and nothing but JUNK.
(I know adsense, but due to my genre they banned me, so screw them.) Currently using Lijit/sovrn but their fill rates suck at about 25%.
Contextweb after 8 months of nice income decided that my website was not interested by their advertisers (total BS) and asked me to stop using them.
Just soo frustrating.. I run a perfectly fine and legal Firearms and Hunting, Politics related site with a ton of traffic and it is highly under utilized money wise.
At least Buysellads is working out well and local advertisers.
Latest disappointments :
lijit/Sovrn : Good ads (sometimes a few bad ones get through though), decent CPM, very low fill rate, counts about 30% of actual requests/impressions vs verifiable GA data.
EPOM : total crap ads. Almost 100% redirects on Mobile devices, horrible CPM
Buysiteads : seems to be run by one or two people. Never answers questions, site seems buggy (by their own words) trying to add a site for approval.
Clicksor : pop-ups and tons of redirects .
Chititka : low CPM that starts to build up, but pop-up ads, fake links, download links to junk and spam/viruses.
smowtion : low cpm, lots of junk ads with fake links to downloads and trojans
Avantlinks : proved sales, but got no credit / income from them, so bad in my book.
I will be testing ADbrite, maybe bidvertiser when I feel persuaded to.