PKT Control, with PKT2 and just regular PKT, is what makes a good or bad ness. If you can't go wherever you feel like with the PKT, you won't be able to recover or punish properly.
In my past experience, there has been a lot of "learn this angle" type of things when I was trying to play as ness. This doesn't help as much. You need to know how to maneuver the PKT around FIRST! The second part is knowing ness' falling speed and adjusting where you move the PKT so that it meets a part of his body at a specific time.
Now going with PKT regularly, you've got several ways to move it around. I always move it in squiggly patterns unless doing something weird; the path can be altered into an arc back towards ness easily (if you're doing it right) and it just looks cooler. Also, it can confuse some people.
Another thing to do is send PKT to travel along the ground in a way so that it is VERY close but not touching. If you do it perfectly, you can scrape some shields. If not, you can cancel the PKT anytime by directing it downward for an instant. It's safe if you have good reaction timing.
You can move the PKT in circles around an area that your opponent must go through in order to reach you to semi-camp, but it doesn't work very well against most characters.
After that, you can just send PKT straight out and in funky arcs in order to get places. I always like being in absolute full control of the PKT, going with it for every single angle and as close to the desired path as possible not letting go of a direction or over-turning the control stick (just in case I need to adjust before I hit myself or to make sure I don't accidentally loop it so that I completely whiff).
A couple of tricks with PKT:
When an opponent is going for the ledge, your PKT with the tailwhip is awesome; place the PKT around the edge so that the head gets there immediately after the opponent gets his/her ledge-grabbing invincibility and loop the PKT head around so that it goes back to the stage for whatever needs to be done. This will basically force the opponent into rolling, jumping, standing, or attacking as a getup, b/c the tailwhip will force the opponent off the ledge if they don't get up before the end of invincibility. If you're good at predicting, you can hit them with the PKT every time. If not, you can potentially hit yourself into the ground or tailwhip your opponent while trying to escape to safety. It's basically 100% safe except against characters that have the best getup attacks from ledge (darn DK!)
Another one, if you are partly following the opponent's path of movement (like if they are falling, go slightly downward, if they are jumping stay near horizontal unless their jump is huge), and the opponent airdodges, you can almost always get to them with at least the tailwhip. If your PKT is near them. An airborne opponent is much easier to get with PKT than some people think. You just have to make sure that the PKT is always following them in some fashion. That's why Uthrow > PKT isn't really good on most characters; they can fall around/through the PKT as it is going up to reach them. Dthrow works okay b/c it keeps the opponent near you so that the PKT can catch up with them faster, but the problem is that many characters can simply use an aerial to get at the PKT and punish you severely (or just take the PKT hit, SDI toward you and hit you with some aerial in your lagging animation).
If you manage to pop someone up in the air with an AC'd Dair at lower percents, you can EASILY lead into PKT. That's sort of why Ref or some other people were looking into AC'd Dair > PKT2 (looping PKT around an opponent) to see if there could be an easy kill. I'm gonna try looking into this and getting a percent range, but SCHOOL HAS IT'S LAST DAYS TO TORTURE ME!
When PKT2ing, I'll leave some must-be-able-to's for you:
90 degrees upward from horizontal
directly horizontal
45 degrees upward from horizontal (looping from EITHER direction and going around ness in either 135 degrees or 225 degrees)
Partly below horizontal
10 or less degrees from upward vertical (to get ledges like SV when you're under the stage)
Also, not by angles, but be able to COMPLETELY recover if you miss hitting yourself the first time. I'll post a video explaining this. It is SO epic seeing people go like "hahahah -- WHAT??!?!" as you recover completely unexpectedly when they think you're dead.