I am a private music tutor and have found that there are times when I only need one staff line to demonstrate something only to wind up wasting an entire sheet of manuscript for the sake of using only one line. Then notes are scribbled between the other staff lines and it all just looks a mess.
Now I go into lessons with a few sheets of blank paper. If I need a staff line then I use the noligraph and Bob's your aunty's brother! It's there and there's plenty of room for other notes.
Just keep in mind, the pens are regular ball-points. As such they don't work so well when it's cold. If you know you're going to be using this, keep it in a pocket until you need it. Otherwise, it will take an extra run or two to get all 5 lines in place.
If there's anything to grumble about it's the price. However ultimately you are paying for convenience more than anything. For a lot of people, this particular product is more of a novelty than anything else.
Regardless, I do love it and am using it daily. I think I'll include one of these in all of my orders from here on in.