We used appengine at a major european car rental company I was working for back then. We deployed a one instance mirror/backup of our booking app which basically cost us nothing. In case our servers, which we kept inhouse, were somehow unreachable we would forward all traffic to appengine and scale up accordingly. After the issue was resolved we would use various mapreduce jobs to migrate new bookings, cancellations etc. back to our servers. The app was written in python with flask on top. To store data we used the NDB Api which was a blast to work with. Overall a really nice experience.

You can find a list here - https://cloud.google.com/customers/

Tried it. High costs, technology lock-in burnt my fingers. Moved to open technologies and not "platforms".

I used Appengine to host a static website for free, a immigration law firm http://www.cbulaw.com/. I built it with Django and exported all pages as static files.

I used it in the past when I learned Python - I built small apps that were just for fun.

It's a powerful engine, and has a lot of nice features.

I'm pretty happy with starting there, because I moved on to Xcode afterwards.

I gave it a try, but I found it too annoyingly restricting. I'd rather just roll my own linux servers since I grew up (I know I'm young) with Fedora Core 2.

Gave it a try but gave up. Looking at compute engine.

I use it. I like that I don't have to worry about scalability and administration is a breeze. I'm happy with it! Been using it since 2009.

Does the fact that no one has left any comment yet indicate that not many people here use app engine?

Khan Academy uses it for their entire site.

Does it support Ruby?

Not directly, but I understand that JRuby can be used on the Java runtime.

Python, Java, Go-lang and now apparently PHP.