Choosing between immutable objects and structs for value objects

Question

How do you choose between implementing a value object (the canonical example being an address) as an immutable object or a struct?

Are there performance, semantic or

Answer 1

From an object modeling perspective, I appreciate structs because they let me use the compiler to declare certain parameters and fields as non-nullable. Of course, without special constructor semantics (like in Spec#), this is only applicable to types that have a natural 'zero' value. (Hence Bryan Watt's 'though experiment' answer.)