I use aggregation queries regularly and it's amazing. My MongoDB database has 4k ops/sec and I run an aggregation query to count averages, sums, and splits on a

I use aggregation queries regularly and it's amazing. My MongoDB database has 4k ops/sec and I run an aggregation query to count averages, sums, and splits on a name (to get averages and sums for each of two dozen groups) for about 10K rows every 5 minutes. Not a huge amount, but this is on a db that is performing a ton of work. Also, I should mention that you should probably run your aggregation queries on your secondaries.