I finally got a chance to try Mongomatic on a flight to Dallas. Overall, I am quite impressed.
MongoDB out of the box has a very simple API for inserting, updating, querying, and modifying data. It is very lightweight and in most cases, extremely readable.
Mongomatic is an extremely lightweight addition to the standard Ruby driver and provides quite a few syntactical niceties which make working with MongoDB even nicer (callbacks, validation, and great modifier support to name a few).
The primary design goals of Mongomatic are simplicity and adherence to MongoDB conventions. Mongomatic is not an ActiveRecord clone and has been designed to fit nicely with the document-oriented nature of MongoDB.
I am definitely still a fan of MongoMapper and Mongoid as well, but the budding minimalist in me is absolutely in love with the with the barebones approach.