I'm using MongoDB as the default data store for a new geo LBS startup. Scalability and performance features were the main attraction, but also I just like the interfaces and ergonomics it presents to me as a programmer. Much prefer representing stored data structures in JSON, doing queries in RESTful HTTP-ish friendly form, and doing processing in Python, JavaScript, etc. Oh and not having to care about data schemas. At least early on in the project lifecycle, when there's no revenue and you may need to frequently evolve and/or significantly pivot the codebase. I love the fact that it tries to keep everything in memory if it can, and that it was designed from the ground up to be distributed.
MongoDB is also rapidly becoming my "goto" data store when making prototypes to explore some new software project space. Because it's so amenable to rapid development and change. It's also replacing memcached in some situations where all I needed was a dumb memory cache. It can act like a dumb memory cache, except it has these extra features waiting in the wings I want, which is a bonus.
I liked Redis in my evaluations and may use it more in the future but it lost out to MongoDB for the LBS project because it didn't fit the requirements as well or get enough little "taste wins" with me.
MongoDB is also rapidly becoming my "goto" data store when making prototypes to explore some new software project space. Because it's so amenable to rapid development and change. It's also replacing memcached in some situations where all I needed was a dumb memory cache. It can act like a dumb memory cache, except it has these extra features waiting in the wings I want, which is a bonus.
I liked Redis in my evaluations and may use it more in the future but it lost out to MongoDB for the LBS project because it didn't fit the requirements as well or get enough little "taste wins" with me.