I also used to have declined payments every once and then, however lately it's been great
Also unpaid bills doesn't affect the application, that's a huge plus
I remember the times when dedicated server providers hibernated the servers for any reasons they like, so overall appengine is great
Generally financial systems decline payments from frequent sources, they probably want to prevent fraud, I think appengine's issue is that it doesn't automatically retry
But this might be a good thing, because if appengine automatically retried those payments, you could have an automatically blocked CC in your hand