We recommend all new apps start using Mango as a default.
What I'm essentially after is on the one hand some insight regarding Mango and on the other hand, an overview of how Mango and Map/Reduce are supposed to live together in the 2.x era.
I recently tried to switch my app over to using Mango queries, with the result of scrapping it completely and switching back to map/reduce. Here are a few of my reasons:
Your mileage may vary, but just wanted to leave a warning that this is still quite new features.