I received a beta invite today and had some time to read the documentation and start experimenting with it tonight.
First, the best thing about Google Cloud SQL: when you create an instance you can specify more than one AppEngine application instances that can use it. This should give developers a lot of flexibility for coordinating multiple deployed applications that are in an application family. I think that this is a big deal!
Another interesting thing is that you are allowed some access to the database from outside the AppEngine infrastructure. You are limited to 5 external queries per second but that does offer some coordination with other applications hosted on other platforms or host providers.
Their cloud SQL service is free during beta. It will be interesting to see what the cost will be for different SQL instance types.
It was very simple getting the example Java app built and deployed. I created a separate SQL instance (these are separate from other deployed AppEngine application instances), made a new IntelliJ AppEngine project, pasted in the example code, and it all worked.
Perception of quality is often influenced by price. Since developers now have to pay more for using AppEngine, I find myself looking more at AppEngine as a premium service, which it is. Despite my dislike for MySQL (I use PostgreSQL when given a choice), Google's hosted and managed MySQL cloud data service looks good and provides developers with more options. Their SQL service is synchronously replicated between data centers automatically for you.
It has been a few years now since I had to either set up a physical server or a leased raw server for any deployments. I like that! Thank you Platform as a Service (PaaS) providers like Heroku (built on AWS) and AppEngine - they are the future. I still do a lot of work on "plain AWS" but that is still much more agile than provisioning my own servers.