We've been pretty happy with the scalability of Mongo where we use it for RowFeeder. We've got over 1 billion documents over several terabytes. The main challen

We've been pretty happy with the scalability of Mongo where we use it for RowFeeder. We've got over 1 billion documents over several terabytes. The main challenge we encounter is our indexes are much larger than RAM, but given that we index by date this isn't a huge problem. Inserts are still very fast and only slow down if we have to load old data. 1.8's ability to distribute reads to replica slaves will help with this a lot, though.