Map/reduce gives an edge as we don't need to process the info again in PHP. For example we want to know how many users are searching in each platform. Either we run multiple queries and get result or one single map/reduce command.
Due to some technical limitations and to ensure our search speed is not deteriorated, we decided to log all requests conditionally and then asynchronously parse and insert them in MongoDB. My next blog post will explain this better.
I have not given a whirl at Aggregate Framework, looks interesting, will try it out.