i think a lot of the reasons I've seen come down to business reasons, not technical. Someone wants an app fast, like now, and MongoDB is fast to setup and get running with.
I guess I'll be able to confirm once I'm forced to build something in it, but I don't think it can really be faster to setup and get running with than CouchDB.
Usually first thing you need to do is write a REST layer on top of it, and with CouchDB that part is just done already.
Obviously there's certain kinds of data I wouldn't put in Couch, or any kind of NoSQL database.
You need to know what the right tool for the job is,but I just want to figure out when that tool is mongo.
Why would that tool ever be mongo ? I think Mongo is a thing because people coming from Rails ORM libraries feel like "wow I can jump on the no-sql bandwagon just by using a library that feels kind of like the ActiveRecord I'm used to".
It's only popular because there's less of a conceptual gap between mongo and the relational database tools that a lot of people are used to.
Couchdb on the other hand requires you to actually learn and use map/reduce.. which is a pain for people who don't feel like having to learn something new, but Couchdb is MUCH MUCH better in a lot of ways and Mongo is pretty much fundamentally flawed in my opinion
I do wish rackspace luck though with their offering. I think it was smart of them to create this mongodb product for one simple reason: a good number of people are already using mongodb so it makes sense to help them get the most out of it.
Usually first thing you need to do is write a REST layer on top of it, and with CouchDB that part is just done already.
Obviously there's certain kinds of data I wouldn't put in Couch, or any kind of NoSQL database.
You need to know what the right tool for the job is,but I just want to figure out when that tool is mongo.