itemsproperty. The ideal use case would be when you need access to the combined value of some ActiveJS constructs as an Observable.
value()method when the Cluster contains immutable Units because when calling the
value()method of a Cluster, it internally calls its members'
value()method, and if members are Immutable Units, those Units would create a clone on every call. It's not a bad thing if used intentionally, but it can add extra overhead if used too often.