I've used them for clients on two occasions. One campaign was for a movie review blog and the other was for an academic services site. One worked REAL WELL. The other was flat. Here's two lessons I can share with you should you try Adbrite:
1) Focus on the level of THEMATIC SIMILARITY between the site you're advertising and the site that will be sending the traffic. The analysis should not stop on the category level (ie., movie site to movie site) it should also examine the similarity of demographics and whether your site COMPLEMENTS the content of the site sending the traffic.
2) As much as possible, optimize your conversions one publisher at a time then gradually add on. For the second site I mentioned above, the client wanted a "basket" of sites sending traffic. I advised otherwise. Instead of learning conversion patterns one publisher at a time, the project bled money. Start slow then ramp up.
Good luck!