In testing of the new up-coming platform, that was a huge, huge win for speed. And we're a Postgresql shop too.
MongoDB allowed us to: * Have embedded documents (very large performance improvements) * Have arrays and hashes as "columns"
We also use Redis in a few crucial places, because of its really good support for lists (queues), and sets, besides, just its blazingly raw speed.
Downsites? Yes. Many rails-style plugins don't work well. But an upside is that we're forced to write leaner code and not depend too much on those.
Another downside, MongoDB is super-fast, but is still a work in progress in some places, and the ORM we're using (mongo_mapper) is somewhat of a moving target right now.
But hey, thats what happens when you're on the bleeding edge.
MongoDB: * build-in replication * basic sharding * embedded documents * very very fast