This design sacrifices flexibility and control by reducing options, but is in line with App Engine's ultimate goal of appealing to developers who want to create and deploy their application quickly, and are happy to have Google handle the rest.

The App Engine design has clearly struck a chord, and it's clear Google now has its hands full, given the service's popularity.

In e-mail interviews with App Engine developers, two things became clear: They are very enthusiastic about the service and they are eager for Google to enhance and expand it.

Some of the most common pluses mentioned by App Engine developers are its automated load balancing; its highly scalable, Google-designed data store, file system and query language; its tight component integration; and overall simplicity.

Bill Katz, a Palo Alto, California, web entrepreneur and developer, already wrote an application called Bloog, which he released under an open-source license, and is working on developing two other applications.

Since his team is made up of just himself and his wife, he values App Engine's integration, automation and simplicity.

"App Engine removes a number of tasks I'd have to handle for a very scalable web app, like maintaining multiple servers [and] a scalable database," he said.

He finds Amazon AWS "great" -- he uses its S3 service for his personal backup storage and might use EC2 to host some Web services that App Engine can't support -- but recognizes it targets a different market.

"AWS provides raw components that can be put together in a number of ways to make a scalable web app. Google App Engine, on the other hand, provides a fully integrated stack that requires you to work a certain way," he said.

Still, it's clear that the enthusiasm over App Engine's attractive features is generating increasing pressure on Google to fill in the service's gaps.

For all his praise for App Engine, Katz misses some key features, such as an expansion of supported programming languages beyond Python.

"It [also] goes without saying that developers are anxiously waiting for the quotas to be lifted and pay-as-you-go initiated," he said. "Background tasks would also be great."

Currently, App Engine applications can use up to 500MB of persistent storage and enough bandwidth and CPU for 5 million monthly page views, although Google plans to allow for purchasing of additional resources at some point before the year is over.

Ben Bryant, president of First Objective Software in Alexandria, Virginia, is in the early stages of developing a commercial application for hosting collections, but won't be able to roll it out until Google gives App Engine a comprehensive, full-text search capability. "Everything else I need is already there," Bryant said.

Filip Verhaeghe, CEO of Self-Star, is also planning to deploy a commercial application on App Engine, but misses secure communications capabilities, which has forced him to come up with in-browser custom encryption schemes. "Hardly the kind of measures that instill trust with customers," he said.