When should I use an OpenStruct instead of a Hash?

OpenStruct objects are useful when you need something to fit a certain method call interface (i.e. send in a duck-typed object responding to #name and #value), or when you want to encapsulate the implementation details, but also want to avoid over-engineering the solution. They also make an awesome stub object, and I often use them in place of framework stubs when I don't need the overhead of a stub/mock.

I think this mostly comes down to a performance decision. From the Ruby Documentation:

An OpenStruct utilizes Ruby’s method lookup structure to and find and define the necessary methods for properties. This is accomplished through the method method_missing and define_method.

This should be a consideration if there is a concern about the performance of the objects that are created, as there is much more overhead in the setting of these properties compared to using a Hash or a Struct.

Additionally, something like a Hash has additional functionality with all of the methods it provides (has_key?, include?, etc.). The OpenStruct is a very simple object from that standpoint, but if you don't have any concerns from a performance standpoint and just want an easy object to work with, OpenStruct is a good choice.