Ron,
Yes, I do own a set of those plug cutters.  I've used them quite a bit and did have a small portion break off one (I think it was the 1/4" one), but I had made a lot of plugs without a break and it overheated.  They have served my needs for a cheap price.  I'm not certain about how precise their sizing is.  I've had them for at least 10 years and probably bought them on a 50% off sale so I think I've gotten my money out of them!
L/W

“Those who would give up essential Liberty, to purchase a little temporary Safety, deserve neither Liberty nor Safety.” Benjamin Franklin