Funny thread!

MongoDB's query performance is typically sub millisecond, and rarely as much as 5ms. I don't think you can through the postgres/mysql parser in 5ms, much less optimizer, planner, and execution stack. Couple that speed with a dead simple API, and you've got a thing of beauty.

So yeah, if you don't care about ms latencies, if you've got a fixed rectangular schema, if want to write queries with lots of joins, if you need ACID guarantees ... etc ... then by all means pick your favorite RDBMS.

OTOH, when you need to manipulate relatively self-contained objects quickly, when those objects don't have a fixed schema, when availability is more important to you than immediate consistency, then why would you choose anything other than MongoDB?