MongoDB is not something I have a good handle on yet. What's its sweet spot? Where should I consider using it instead of Postgres?

If you are working with location data, Mongo has built in geospatial indexing built in since 1.4 (earlier in the unstable builds) - http://www.mongodb.org/display/DOCS/Geospatial+Indexing which has been a big draw for a number of people I know using it (it looks like 1.8 brings spherical distances to the stable branch which makes the geo lookups a lot more useful if you need accurate distances and not just near by lookups).

I got excited and looked it up.

> We don't currently handle wrapping at the poles or at the transition from -180° to +180° longitude, however we detect when a search would wrap and raise an error.

generalized grumble

Why does everyone always seem to punt on doing geospatial right? It's not _that_ hard.