public class MyCoolViewModel
{
    string _FooBar;
    public string Foobar {
        get { return _FooBar; }
        set { this.RaiseAndSetIfChanged(x => x.Foobar); }
    }

    public MyCoolViewModel()
    {
        IObservable<ObservedChanged<string>> changesToFoobar = this.ObservableFromProperty(x => x.Foobar);
        changesToFoobar.Where(x => x.Value != null).Subscribe(Console.WriteLine);
    }
}