I'm using GAE for QueryTree (http://querytreeapp.com http://querytreeapp.com) which is a data analysis tool. The GAE data store is really nice for persisting o

I'm using GAE for QueryTree (http://querytreeapp.com http://querytreeapp.com) which is a data analysis tool. The GAE data store is really nice for persisting objects that come from users filling in HTML forms, or equivalent. One user can only type in data so fast and that kind of volume is fine on GAE. Plus you get great read scalability. However, if you're creating data in some sort of simulation or loading it in from elsewhere, use a relational database (you can connect to external servers from your GAE app). Loading a few million rows into the GAE data store would cost you a fortune and the lack of joins or proper SQL probably means you have to get them all back out into memory again before you can do anything useful. QueryTree uses the GAE data store as a central hub for user account info and settings, then shards lots of MySQL instances for actual data work, which seems to work well.