I agree that document-oriented databases will probably not replace relational databases in the near future. In my opinion though, the schemaless design of Mongo

I agree that document-oriented databases will probably not replace relational databases in the near future. In my opinion though, the schemaless design of MongoDB paired with its ease of use and its native support of JSON data makes it a perfect choice for prototyping and (in some cases) a viable option for use in production. What you also have to consider when comparing document-oriented to relational databases is that the former is still a very young technology: MongoDB has been founded in 2007, whereas Postgres has been around since 1986! So given what the MongoDB team has achieved in such a short time span, I expect to see some huge improvements in this technology over the next decade, especially given the large amount of funding that 10gen received. In addition, the root cause for most complaints ("it doesn't scale!", "it loses my data!", "it's not consistent!") is that people try to apply design patterns from relational databases to MongoDB, which often is just a horrible idea. Document databases and relational databases are very different beasts and need to be handled very differently: Most design patterns for relational databases (data normalization, using joins to group data, using flat data structures, scaling vertically instead of horizontally) are actually anti-patterns in the non-relational world. If you take this into account I think MongoDB can be an awesome tool.