My approach was to read the Dynamo paper. It's presents the goals of Dynamo and the technology from which it is constructed in a very compact form. From there you have a basis for understanding the engineering tradeoffs made by the other NoSQL databases. Then you can actually make decisions based on what fits your use case, rather than just choosing Mongo because it's popular. (Don't get me wrong, Mongo is great in its niche.)

Reading the Google BigTable paper would also be a good idea, as it represents another major strand of work.

My blog post on Dynamo: http://untyped.com/untyping/2011/01/21/all-about-amazons-dyn...