The database is still aware of the fields, so MongoDB can build indices on certain fields if you wish. Admittedly I haven't deployed Mongo in an environment tha

The database is still aware of the fields, so MongoDB can build indices on certain fields if you wish. Admittedly I haven't deployed Mongo in an environment that really tested its performance, but we've been serving about 20k pageviews per day with no issues. Granted, this was a fairly basic application. As for changing requirements, mongo handled those well too. It's certainly not a silver bullet, but when I just need a basic object store the query performance trade-off is worth it.