Sample of a viewmodel with Reactive UI

App.xaml

<Application x:Class="WpfApplication1.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             StartupUri="MainWindow.xaml" xmlns:local="clr-namespace:WpfApplication1.ViewModels">
    <Application.Resources>
          <local:ApplicationViewModel x:Key="MyApplicationViewModel" />
    </Application.Resources>
</Application>

MainWindow.xaml

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525" DataContext="{Binding NewUserViewModel, Source={StaticResource MyApplicationViewModel}}">
     <Grid Name="MyGrid">
          <TextBox Name="Input" HorizontalAlignment="Left" Height="23" Margin="93,111,0,0" TextWrapping="Wrap"
                    Text="{Binding Path=Password, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" VerticalAlignment="Top" Width="120"/>

          <Label Content="{Binding Path=Password, Mode=TwoWay}" Height="30" Width="100" Margin="93,154,324,136"></Label>
          <Button Content="Button" HorizontalAlignment="Left" Margin="218,112,0,0" VerticalAlignment="Top" Width="75" Command="{Binding Path=OkCommand, Mode=TwoWay}" />
      </Grid>  
</Window>

ViewModel.cs

public class ApplicationViewModel : ReactiveObject
     {
          private NewUserViewModel _NewUserViewModel;
          public NewUserViewModel NewUserViewModel
          {
               get { return _NewUserViewModel; }
               set
               {
                    this.RaiseAndSetIfChanged(x => x.NewUserViewModel, value);
               }
          }

          public ApplicationViewModel()
          {
               NewUserViewModel = new NewUserViewModel();
          }
     }

     public class NewUserViewModel : ReactiveObject
     {
          string _Password;
          public string Password
          {
               get { return _Password; }
               set
               {
                    this.RaiseAndSetIfChanged(x => x.Password, value);
               }
          }

          public ICommand OkCommand { get; set; }

          public NewUserViewModel()
          {
               var canHitOk = this.WhenAny(x => x.Password, (pass) => (pass.Value != null && pass.Value.Length > 3));

               var cmd = new ReactiveCommand(canHitOk);

               cmd.Subscribe(_ => MessageBox.Show("Wee: " + Password));

               OkCommand = cmd;
          }
     }