Depending on your needs, mongo has a much nicer interface than redis... Not to mention supporting indexing in the DB instead of having to DIY your own... I used it at a site that did mostly classified listings, and the performance was a lot better than the SQL RDBMS that was being used for search/display.
Yes, I agree. But regarding my comment, the use case was caching with persistence. Redis has some persistence options (pure snapshot or pure journal), but they just aren't its primary use case and aren't nearly as mature or performant as MongoDB's memory-mapped filing. Also Redis is key-value so requires manual indexing, i.e. much more up-front developer effort.
