Don't use Lijit, ever. I was running them as backup to my Adsense, sending them nearly a million impressions a day. They run all sorts of malicious advertisements, and apparently they approve of them. My users were getting redirected from my site, to sites telling them their Javascript is outdated, with download links to malware. My mobile users would get popups about their Android device being out of date, and similar. I looked up the redirect URLs on Google, and this was affecting other sites as well, and I found a handful of forums running Lijit, where their users were reporting the same troubles.
This was incredibly damaging to my reputation. I narrowed it down to Lijit after toggling my ad networks and informed them of the issue. I emailed them, explained how my users are getting these redirects on a daily basis, how this is unacceptable, and how I'll have to pull their ads until they're able to resolve the issue. Their response? They didn't care, they just said, we understand your concern for not wanting to continue our relationship. That was it, they're not looking into it, they didn't apologize, nothing.
I tried them again last month, after I cut them for the previous 6 months. Within a day, I had over a dozen users sending reports about malicious redirects. I disabled Lijit, and things were fine. I used Lijit for 8 months without trouble, and enjoyed their service since the interface is decent, rates competitive, tracking and payments done right, etc. However, since these issues started, I have no choice but to leave. It's sad they have no interest in solving them, because like I said, they have potential. Redirecting my visitors to malware though, and allowing that on the network? It's inexcusable.