I used Mongo in Python for a couple of projects recently. I tried out every single library I could find. In the end I settled on using Schematics [1] with a very thin data layer for loading and saving the objects. It worked really well. You get to work with sane Python objects and it's really easy to move between Mongo and Python.
Ultimately, two projects later I'm just not happy with Mongo as a data store. I've found that the restrictions of a relational data model mean that you don't end up paralysed worrying about how to store your data. In Mongo I was forever tossing up between embedding data or splitting it up into a separate collection. And it's a choice that may well bite you later. Also, only ensuring the integrity of your data within the application is not a great idea.