The difference between ¿Cómo fue tu viaje? and ¿Cómo estuvo tu viaje? is subtle, as both mean “How was your trip?” But fue (from ser) implies asking more about the overall nature or essence of the trip, while estuvo (from estar) can imply a question about specific aspects or moments of the trip. Both are used interchangeably, although *fue* often feels a little more general. @rojocello