I have tried to use it now. There are lots of work for them to do before it is good. Intellisense is now working. That makes it a lot easier to get the XAML right. Many controls are not ready. Datagrid is not ready to be used. I had to comment out most triggers. I am converting a WPF application to WinUI 3. XAML designer is not working. To open a FileOpenPicker you have to write some strange things.
var Dlg = new Windows.Storage.Pickers.FileOpenPicker();
StorageFile SF;
IntPtr windowHandle = (App.Current as App).WindowHandle;
var initializeWithWindow = Dlg.As<IInitializeWithWindow>();
initializeWithWindow.Initialize(windowHandle);
Dlg.FileTypeFilter.Add(".txt");
SF = await Dlg.PickSingleFileAsync();