We jumped on Appengine very early, and have never regretted it. It's remarkably stable, and not having to worry about the security issue du jour, scaling, or any other sysadmin stuff means we can concentrate on what we do best - building apps.
I've been surprised how few people understand the value proposition, and how little competition there is. When I first heard about Azure I expected it to be PaaS, but it turned out to be Windows-first AWS.
Not as I understand it. Both AWS and Azure force you to use machines - you may put them behind load balancers, and they may start up automatically when required, but you still need to design the system and do provisioning and plumbing to make your application working.
With Appengine, you don't know anything about the hardware of software your application is running on - you simply upload your app.
I haven't used Azure, so please correct me if I'm wrong. I believe AWS Lambda is the beginnings of a "serverless" environment - maybe Azure has an equivalent.
Azure has a ton of serverless components. Functions is their lambda equivelant. Azure has many services you can create apps and capabilities without standing up a sever. Like anything else it has some pros and cons.
I've been surprised how few people understand the value proposition, and how little competition there is. When I first heard about Azure I expected it to be PaaS, but it turned out to be Windows-first AWS.