I run goread (http://goread.io) on App Engine. Doing the math by hand, I'll get a 25% reduction in costs. This comes from cost reduction of instance hours and datastore writes, and free SSL and datastore small ops. I didn't get the advertised 30% because my largest single cost is datastore storage, which didn't drop. I'm happy with how this all went.
I saw another comment asking about app engine pricing. It is priced based on many different usages: instance hours (where instances are automatically added and removed by app engine itself), writes, reads, and storage to their NoSQL datastore (far and away its best feature, which no other provider in the world offers). They charged small amounts for other things, too. I was paying $9/month for SSL and other small one-offs. This new pricing basically eliminated all the other one-offs, making them free. It reduced the number of kinds of things that are billed. Before the datastore had 3 pricing tiers depending on the operation. They made the cheapest free and the other two the same price (reducing the price of the more expensive). Overall things got much simpler and cheaper.
I saw another comment asking about app engine pricing. It is priced based on many different usages: instance hours (where instances are automatically added and removed by app engine itself), writes, reads, and storage to their NoSQL datastore (far and away its best feature, which no other provider in the world offers). They charged small amounts for other things, too. I was paying $9/month for SSL and other small one-offs. This new pricing basically eliminated all the other one-offs, making them free. It reduced the number of kinds of things that are billed. Before the datastore had 3 pricing tiers depending on the operation. They made the cheapest free and the other two the same price (reducing the price of the more expensive). Overall things got much simpler and cheaper.