Hmm, I use the film and like it alot, as do other people I know. I wonder how you are holding your ruler? Some people want to use the flat of their hand, which allows the ruler to slip around more. I teach my students to keep their hand in an upright position with just their finger tips pressing on the ruler, with their pinkie on the outside edge of the ruler pressing on the mat. Also, never cut beyond where your fingers on the ruler hand are - stop the forward motion of the cutter, but leave it in place, and 'walk' the fingers of the ruler hand forward to continue a long cut.
I hope this makes sense - it would be easier to show you!