Over the past 2-3 weeks, I've seen fill rates plummet from 99% to around 51% - this was shortly after a period where users complained of ads horribly slowing down the apps.
I have asked AdMob a couple of support questions, and got no reply at all.
Needless to say, I'm not very happy with the company. Is there an alternative way to provide users with free, ad-supported apps?
(related: http://www.anddev.org/viewtopic.php?p=28608 )