Just from my recent Jetstar experience - don't expect the staff to be friendly or helpful, do expect a senior officer with an attitude, do expect cramped seats and do expect the guy a couple of rows back to chunder in the toilet....others may have had good Jetstar experiences, mine were not, even though the flight was only 1/4 full the passengers still seemed an inconvenience.
Virgin blue on the other hand were lovely and that flight was full up.