We had signed on with Adbrite to have our ad shown on different sites. The main problem was that practically all of the websites (approx. about 85%) where our ads where shown were just bogus pages created by publishers to collect clicks. Adbrite did offer to refund the difference, which was great, however, I'm not so sure that they are actively looking to remove these scam sites from their network. Another problem is that when I tried to pause the ad campaign, it didn't pause. They charged us a higher amount than they should have, and now they're refusing to refund us the difference. I just wanted the readers of this forum hear my story and make up their own minds about Adbrite.
There's one rules with advertising with Adbrite is for you to manually select each site that you want to advertise on. When ever I have used them I have only selected the high alexa ranked sites that I know are good and I also check how many ads they have on their pages, how they have the ads, etc.
Yes, you can get quality traffic from Adbrite, but it does take some amount of work. I never buy the network ad option. What I do is when I browse the site directory, I sort by lowest CPC because sometimes I get lucky and buy an ad for $3 or so (for a month) and end up getting way more traffic that it stated that I would get. I also go to each site that I'm considering and look at where the ads are displayed to see if they are in a prime location and to make sure the site is not crap. Hope this helps!