On my site interest based ads appear to pay twice as much as contextual, but the CTR is about a quarter. I opted out of IBA a couple of days ago, but I'm unclear as to whether or not that stops the pesky interest based ads following you around.
I know personally I find it irritating and I can't help but wonder if visitors are seeing the same and therefore not seeing ads related to the page content they are looking at. I'll leave it turned off for a period of time and see how things pan out.
Very useful to see this new metric on our reports though so thanks for that.