1) They’re very powerful, but very annoying, so they should be used very sparingly, and only when there’s a good reason. This is the first cliffhanger I’ve done in CP, and it seemed to me that the question of how Martin could manage to get a job offer from a major airline, given his particular strengths and weaknesses; and the question of what he would do if he got such an offer were both too big to be dealt with in a single episode. Plus, the issue of Martin’s need to be paid to do the job he loves versus Carolyn and Douglas needing him to go on being unpaid in order to make MJN viable has become the central dilemma of the whole show (It didn’t use to be, but the show has changed). It seemed like the question of whether and how that is resolved was worthy of a cliffhanger.
2) You can’t use a cliff-hanger instead of an ending. Some shows do, but I think it’s cheating. Any episode that ends with a cliffhanger must also have a satisfying conclusion in itself. Ideally, the main question of the episode should be answered - but the answer should then throw up an unexpected larger question, which provides the cliff-hanger. So, for me, the question of this episode is ‘Will Martin get the job, and if so, how?’, and it’s only when that’s resolved that we’re reminded that the bigger question is whether he takes it or not.
3) The cliffhanger has to be an emotional one, or at least a direct dilemma for a central character or characters, not a physical or external one. The question left unanswered must always be ‘What will he or she do now?’ not ‘What will happen to him or her now?’ To take an example completely at random, a bad cliff-hanger would be ‘The hero’s been forced to jump off a roof! Will he survive?’, but a good cliff-hanger is ‘He DID survive! But how? And why’s he hiding from his friend?’ (Oh, but by the way, Steven Moffat is a terrific writer, and it’s an honour to be compared to him. But he did not invent the idea of a cliffhanger ending. Writers have been doing it for really quite some time.) So, in this case, it would have been totally unfair to make the cliff-hanger ‘Will they offer Martin the job or not?’ firstly because it would break rule 3 above, but also because by then it’s out of Martin’s control. But ‘He gets it! Does he take it or not?’ seems to me fair game. Your mileage, of course, may vary… And most importantly of all:
4) A cliff-hanger is a promise to the audience. It’s implicitly saying ‘I’m withholding the gratification of giving you the answer now, but trust me, when you get it, you’ll think it was worth the wait.’ And if you’re going to make a promise like that, you’d better be able to back it up, or at least think you can.