I have the 12"x12" with 1/2" slots and LOVE it.
It's like having training wheels on your rotary cutter.
You can't waver off the straight line.
I make sure I have a finger on the plastic piece beside the slot I'm cutting to stabilize it, otherwise if you press against it , it will bow a bit.
I can cut out a quilt in an hour now , instead of hours and hours.
Love it for making squares, they're right on, everytime.