Often times when we display the value of Checkbox or Boolean Attribute, we happen to see true or false getting rendered as its value. When we need custom values (like Yes/No) to be rendered on the screen, you can use ui:outputText
<ui:outputText value="{!if(v.some_booelan_field__c, 'Yes', 'No')}"/>
This renders Yes when the value of the field is true. Or No when the value of the field is False.
Remember, you cannot use IF (uppercase) in lightning. It throws
No function found for key: IF: Source
James' Desk 