i've been using it for about 7 years now- it cuts down on lint- and makes the thread smoother/glide nicer. it was recommended to me by a (big-name) quilter who has won numerous international awards- i figure if he uses it i should feel pretty safe using it- it has made a big difference.
when i wind bobbins i just put a drop on the thread on the bobbin- and i run a few thin lines of it down the sides of my spools- especially when using cotton thread which seems to create alot of lint- and frays easily