This word is very often used by fans and also by authors in the meaning of "goodbye". Although it has been used in such situations even onscreen, one should not use it as a farewell greeting. Its meaning "success" should always be taken literally. So if somebody leaves a place for a mission, you may wish them success, but they should definitely not answer the same word.
In addition, it makes no sense at all running around at a convention, greeting every Klingon with Qapla'.
Pronunciation
It is important to pronounce the two syllables separately: Qap + la'. Most beginners tend to say "ka-pla", but there is no consonant cluster [pl] in Klingon.