We are using it for a portal that collects data from solar panels (i hope thats the right word) and presenting that data on a website. Not exactly what the drupal developers had in mind for it, I imagine, but the boss found it, he likes it, so we are using it ...
At first i was pretty much impressed by the whole system. Then, after a few days after looking into it and the HORRIBLE things my predecessor 'coded' i got annoyed by the whole overhead and thought the whole project would die within a week. These solar panels send quite a bit of data and with all that data represented as nodes we had about 2,000,000 nodes after only a few month, most of them being utterly useless (Apparently my predecessor had no idea what he was doing). That slowed down Drupal, and, more importantly, crashed the soap-server, which collects the data the panels send and stores it using drupal-functions.
Now after looking into how to write modules and reading a little in this excellent book
I think we can save the project and get a nice, modular and reusable piece of software. I'm impressed again \o/