OOPS, I'm sorry just noticed Mah-jongg that you had already said you have the 550e machine.
I only have used the Monster Hoops at a Dime event we had at our shop. We only got to actually practice hooping with them, no embroidery because the size they brought wasn't the size that fit on our 500 machines.
So, what I can say for a fact is that the magnetism is incredibly powerful, can hurt you if you pinch yourself (!) and will surely hold the thickest of fabrics very securely.
I have a friend who embroiders with a Brother machine and she loves her Monster Hoop.
If Dime could make a Monster Hoop for our 15000 I would sure like to have one, but that won't ever happen.