Cantonese 咩 (me1) vs. 嗎 (maa1) What is the difference between 咩 (me1) and 嗎 (maa1) in Cantonese? Are they always interchangeable? Example:


  m4 sai2 faan1 gung1 me1? (You don't have to go to work)
咩 (me1) as a sentence final particle is used not only to mark the sentence as a question, but also to indicate surprise (or disbelief) that the situation is not what you expected.  Taking your example:


  唔使返工咩？ (m4 sai2 faan1 gung1 me1?) "You don't need to go to work?"


This asks why the receiver doesn't need to go to work, but also indicates that you were originally under the impression that the receiver needed to go.

The use of 嗎 (which is usually pronounced maa3, not maa1) in Cantonese is usually considered formal and not commonly used in everyday speech.  Its existence in Cantonese is most likely due to Mandarin influence.  To express a neutral question, the verb-not-verb construction is generally favored.  Expanding your initial example, if you just want to ask whether someone needs to go to work, you would say:


  使唔使返工呀？ (sai2 m4 sai2 faan1 gung1 aa3?) "Do you need to go to work?"


The sentence final particle 呀 (aa3) is often placed at the end of questions, generally to soften the tone so that it won't be considered abrupt.

For information on these and other Cantonese sentence final particles, check out http://www.cantonese.sheik.co.uk/essays/cantonese_particles.htm

P.S., Fivesheep's answer notes that 咩 is a contraction of 乜嘢 (mat1 je5), but this is a completely different me1 that just happens to be represented using the same character.  The use of 咩 as a sentence final particle is not related to this and is not a contraction of 乜嘢.