depends on the situation and how good you are at them. 99 percent of the time you will prolly not want to throw a flying side or flying 360 but theres still that 1 percent you might just go with it and make it work.
There is a technique I have found particularly effective in sparring in which you feint a kick with one leg and use your mometum to jump and kick out with the other leg. It works well because the first kick lowers your opponent's guard and opens him up to the second kick.
Along the same lines as what Shaolindragon just suggested, I have found that jump snap kicks to the face are extremely effective in full-contact sparring-- they are faster than running side kicks, and they are less easy to see coming than twirlies. Also, the knee up is a feint, and it leaves your opponent's face unguarded, generally.
Flying kick is too obvious, and Im assuming you mean a real flying kick like with a prep and run. You will never land it, unless his already in pain or stunted, but if you do land it then is time to call 911 for your victim.