Over the past 6 months, we've "scaled" MongoDB by moving data off of it. [...] For key-value data, we switched to Riak, which provides predictable read/write latencies and is completely horizontally scalable. For smaller sets of relational data where we wanted a rich query layer, we moved to PostgreSQL.
As usual, take this with a grain of salt (performance varies with data size, use cases, read/write ratio, etc).
I had a short experience developing with MongoDB and I like it. Unfortunately, I had not the opportunity to see how it behave in production use.