I can only talk about my experience, but I've had zero downtime with GAE since we migrated to the HR datastore. Development is x1000 harder, but then everything Just Works.
App Engine's reliability massively improved with the HR datastore, and has gotten even better since the pricing change / SLA guarantee. It's actually remarkably good now, I recommend taking another look.
At the time It was like voodoo, and you had to triple-check your datastore actions, because they could fail for no reason at the backend.