I talked with one of the guys that works for Google App Engine, and he confirmed that BuddyPoke is one of the biggest (if not THE biggest) apps that run on it. That app serves millions of DAUs on various social networks and runs just fine.

I've set up things on app engine and amazon as well. I found GAE just to be much, much easier and saved me a lot of time and money (yes I know EC2 is cheap but it's not cool to see your bill growing when you're just testing things).

I actually took the time to switch to EC2/S3 for a couple days because I was worried that GAE was not really serious about making it a productive place to run serious applications. Portability was a big factor too. But after a while I figured that all this worry was nonsensical. If I ever have to move things over, it must be a good sign and either someone will have built a tool for it or I just spend a week building a tool for it.

Also, re: SSL. You can actually do it right now on GAE but it involves a few tricky workarounds. That said, the GAE team just pushed a more straightforward feature for this to the top of the roadmap. Beforehand they had said they weren't going to work on it anytime soon, but presumably after so many people complained they decided to go ahead with it -- another good sign that at least they're responsive to developers' needs.