I have been developing with MongoDB for about the past year. (Briefly, I have developed, and continue to support, an ASP.NET MVC C# Web Application that's used by a few thousand people daily. For my personal projects that use MongoDB, I develop with PHP and Python.)

Overall, I enjoy working with MongoDB, because it (generally) maps directly to your object - there is no need for an additional layer (such as an Object Relational Mapper (ORM)).

However, you have to be more careful with your data structure. For example, having sub-arrays in sub-arrays is probably not a good idea.

I will be happy to share more, so feel free to look up my profile.