I have a flex shaft unit, re-branded by CC-Specialty, it ported alot of 2-stroke cylinders in it's day.
As stated, it is like a dremel on steroids, I have boxes of 1/8", 1/4", long reach, etc burrs, not much I can't grind on. Plus the ability to change the hand piece is tremendous, still have my right angle hand piece, invaluable for modifying the transfer ports on a 2-stroke.
I've had several offers to buy the setup, can't see that I would ever part with it.