Unfortunately, many of those who used mongodb regretted the decision

Mongodb is a good db but it is not good for many use cases that people are using them for. There are a lot of companies that realized the mistake and are actively migrating off it. I know we are planning a "Off-Mongo party" with another company once we both manage to migrate off

I think you're being too kind.

Many of us who used MongoDB have concluded that there is no task at which it is in any way better than all other existing alternatives.

That's a pretty bold statement. Care to enlighten?

I agree that document-oriented databases don't deserve a lot of the hype they get (use the right tool for the job and all that), but there is definitely some use cases where they make sense.

At my company most of our systems are SQL Server powered, but one of the newer systems that stores large blobs of metadata for products is using Mongo, and it is working quite well.

You are either being disingenuous or you're ignorant.

Many of the companies that switched off MongoDB were growing and ended up moving up to databases like Cassandra. MongoDB is a great database from when you're starting to when you're mid sized.

Cassandra destroys PostgreSQL in scalability but we don't say PostgreSQL is a crap database because of it.

What makes Cassandra unsuitable for starting to mid sized where MongoDB is better?

Calling it new and untested is a bit too much. It's probably more accurate to describe it as being tested and found unworthy for most use cases.

MongoDB is web scale

http://www.mongodb-is-web-scale.com

That has so much win. Thank you.