I've been using Lijit for about six weeks after my other CPM netwrok started having issues. I've found them to very easy to work with, responsive to emails, and pay a decent CPM for my site. They did contact me first via email after finding my site on a search. If you read up on them you'll see they are geared towards the smaller publisher...the one's Federated never would have touched or many other ad networks.
One note - CPM for foreign ads is very low so if you have a lot of visitors outside of United States they might not be a good fit for you.