I use them in all of my vertical bobbin machines - the difference in the stitching on the backs of my quilts is incredible, whether I'm using a 15-91 or a Kenmore from the 50's or one from the 70's - they're just like little miracles.
They don't go into any of the machines I own with horizontal bobbins, though - 66, 201, 401, 500 - nor do they go into the 301.