After my initial love affair with Yahoo! things started to turn south. I had a few absolutely awesome days in which I made roughly triple what I normally would off of that same traffic, but the earnings steadily migrated downwards until they were less than AdSense for the same amount of traffic. The CPM doesn't lie, and I let it earn crappily for better than a week before I switched back.
Now, Yahoo!'s earnins weren't much worse than Google's. In fact, they were really quite comparable, but just not better. Google's reporting is still better and while I do like Yahoo's interface quite a lot, it's not a good reason to justify staying.
If Yahoo! could get the CTR up to Google levels then I'd be making SERIOUS money. The RPC is way high, but the CTR is abysmal.
Iron out some kinks Yahoo!, and I'll consider coming back.