[kujenga]

We use both and try to stick with standard runtimes wherever possible, resorting to flexible runtimes for things doing heavier processing and/or with lots of instances.

The standard runtimes are superior in a lot of ways, two big wins being lightning fast deployments (seconds vs. many minutes) and batteries included for all the app engine services. Logging in the flexible environment can also be a bit lacking. When you have something running in standard runtimes you basically don't have to think about many of the things that can normally go wrong on a server.

That said, many things about the flexible environment are rapidly improving, they've certainly come a long way since we first started playing with them around 6 months ago.