The above solutions didn't work for me for classes with background images somehow. What I did was I create a default class (the one you need in else) and set class='defaultClass' and then the ng-class="{class1:abc,class2:xyz}"
<span class="booking_warning" ng-class="{ process_success: booking.bookingStatus == 'BOOKING_COMPLETED' || booking.bookingStatus == 'BOOKING_PROCESSED', booking_info: booking.bookingStatus == 'INSTANT_BOOKING_REQUEST_RECEIVED' || booking.bookingStatus == 'BOOKING_PENDING'}"> <strong>{{booking.bookingStatus}}</strong> </span>
P.S: The classes that are in condition should override the default class i.e marked as !important