it works alot,many people dont expect.by the time they know what your doing its too late. they get the "KNEE OF JUSTICE" which ruins thier recovery.its a great way to kill.
Plus, even if you flub the knee, just u-air. You can usually up-b back to stage, and gimp them so bad they die.
Other than that, the only way I ever use the knee is to punish landing lag. Or aerial ending lag (LOL ike) It ***** heavies (DK and Bowser, mainly. they has the big hitboxes.)
But for what it's worth, a good way to get to the edge, (only really works if the opponent spams airdodge) dash attack, utilt, dash attack, utilt, etc. Then to the edge, then dtilt. Then kneerape.