MSN has always produced the best audience for me. Surfers arriving from MSN stay the longest and visit the most number of pages in a session. But they just are simply not capable of delivering anywhere near the amount of visitors as Google is. And their PPC campaigns are way more expensive per click too! Unfortunately, even with Google's "lesser-quality visitors" as far as my specific traffic is concerned, it just doesn't balance out, and I have to stick to mostly organic with a little dash of AdWords thrown in.