How appropriate is "拜拜"? How appropriate, in terms of casualness and/or childishness, is "拜拜"? Is it as childish as "Bye-bye" would be in English, or is it more acceptable?

In case location matters, I'm mainly interested in how it's viewed in Taiwan.
拜拜 is not casualness and/or childishness,it is a more friendly way to express goodbye ,and majority use between people who have good relation,or see somebody for the first time and build a friendship each other.
So does it in Taiwan I think:)