How to implement correctly IUserType?

I need to create a custom type for NHibernate by writing a new mapper class that implements IUserType. While it is relatively straightforward to override most of the methods and properties, I get some difficulties to understand how to deal correctly with the following members:

object Assemble(object cached, object owner);
object DeepCopy(object value);
object Disassemble(object value);
object Replace(object original, object target, object owner);

I do not understand what is exactly their purpose; and more important, how to properly implement them. Most of the examples I have seen just return the raw input parameter.

public object DeepCopy(object value)
{
    return value;
}

public object Replace(object original, object target, object owner)
{
    return original;
}

public object Assemble(object cached, object owner)
{
    return cached;
}

public object Disassemble(object value)
{
    return value;
}

How to implement those methods correctly in a real case or more complex scenario?

Solution

Have a look at how Ritesh Rao has done this in his NCommon framework:

The MoneyUserType implements a base class called CompositeUserTypeBase

There's more detail in the comments but to summarize:

DeepCopy - should return a deep copy of the persistent state, stopping at entities and at collections
Disassemble - transforms the object into its 'cacheable' representation (i.e. associations must be cached as identifier values)
Assemble - reconstructs an object from the cacheable representation

It's explained in a lot more detail here