I use the hell out of it, though not as much as I should.  When I'm deploying ads on a new page I always make sure that each ad spot has 5-6 niche targeted ads in rotation, then I leave them for a month.  When I come back to check ad performance 2 or 3 weeks  later I look at CTR for each ad spot, and I usually have one ad performing great, 2-3 performing OK and 1-2 that are for shit and get dropped.  It's help me fine tune traffic to sponsors immensely 
