Consider that my ViewModel has the IsBusy propery. It was great if I was able to do binding like this:

<ProgressBar Visiblity="{Binding IsBusy}" ... />
<Button Visiblity="{Binding !IsBusy}" .../>

Note that I've bound Button's visibility to !IsBusy (Not Busy).

I used to have this on JavaScript MVVM frameworks like knockout.

The expression could be more complex like:

<Button Text="IsBusy && !HasError ? true : false"/>

Currently I should use ValueConverters for this which is hard to create, for each type of binding required.