Cheating? In what sense?
In the sense of letting you get more numbers with less work from the upper body? Absolutely. However, the assistance doesn't come from a gravitron or a JS band here. It comes from your hips. And that means that you're doing some extra work while kipping by getting the whole body into the pull-up?
In the sense of retarding your progress? No. When you cheat by spastically kicking your legs, not going to full extension at the bottom, not getting your chin above the top, and so on, you compromise your progress. Doing those kind of cheated reps doesn't even help you do more CHEATED reps as much as doing reps with good form would, let alone help you increase your number of reps with good form. Kipping is not cheating in this sense: Your kipping pull-up numbers will go up, and your dead-hangs will go up with them.
Are they as good as dead hangs for developing the ability to do dead hangs or weighted pull-ups? I don't know. Some people have claimed substantial weighted pull-up gains from just kipping pull-ups (e.g., from BW+100 to BW+140 or something like that), but I haven't seen any weighted pull-up gains. But then I didn't see any from pure dead hangs training either.
As I see it, the only completely persusasive argument that you should kip is that it allows you to squeeze more total effort into a given time period when you're doing a circuit of multiple exercises, because you're using your legs, too. Forget about all the power calculations for a moment and just ask yourself which is more of a puker: A circuit of front squats and military presses, or a circuit of front squats and push-presses? That's the real reason for kipping.
Form: Kipping is damn complicated. Diffeent people seem to do it in different ways. Lots of people sort of rock on the bar (round back - arched back - round back - arched back). The best guys and girls (the ones with 50+ pull-ups) seem to do more of a frog kick thing, bringing the legs rapidly up and back.
I have the up part of the kip down, but the eccentric still escapes me. The trick seems to be to push the bar right before the top of the pull, almost as if you were doing a steep incline bench, and then somehow get your hips back into position to kip again. It's the hip reset that's got me right now. I'm at 28 now, could probably crack 35 if I could push the bar away on each rep instead of locking at the top and lowering.
If you want to learn, watch lots of the videos posted to the CrossFit front page and front page archives - check out all the ones that mention pull-ups (duh) and also all the ones of the workout 'Fran' (which includes lots of pull-ups).