I've twice tried (unsuccessfully) to use it for small projects. Last week I had to re-write an app to port it off the platform because it suddenly stopped being served correctly when accessed via my own domain name.

Basically it stopped working, no error messages, no way to debug the problem.

I like the principle behind GAE and friends, but I keep going back to hosting things on my VPS. It works, and when/if it breaks I can surely fix it.