MongoMapper is very easy to learn because it mainly respects the ActiveRecord API. You will not be lost in a unknown world because you will be able to manipulate your documents as you are currently doing with your AR records. You can persist documents using well-known methods like save, create and update_attributes. You can also select the data you need using finders and conditions. Associations, callbacks and validation are available as well. In fact, the views and the controllers generated by a scaffold can directly be used as they are with MongoMapper.
This blog is called myNoSQL and it is written by me, Alex Popescu, a software architect with a passion for open source and communities.
It records my readings, learnings, and opinions on NoSQL databases, polyglot persistence, and distributed systems -- subjects that I'm passionate about.
The opinions expressed here are my own, and no other party necessarily agrees with them.
If you feel I'm biased, I probably am.