My wife is an avid quilter. Awhile back she gave me her extra rotary cutter to see if it was of any use in modeling. It is!!! Yesterday I was trying to cut some small pieces of sheet styrene, struggling to hold the ruler and styrene in one place, and decided to try the rotary cutter. That thing works like a champ. You just hold it up against the ruler, roll it back and forth with a little pressure, and the piece of styrene is accurately cut off in a few seconds. You don't even need much pressure. and the blade I'm using is a used blade--one she didn't mind letting go of. You might check out one of these online if you do any scratch building or kitbashing.
I used to have a similar one many years ago. Picked it up at a yard sale. Worked great but it got lost.
You can get spare blades on ebay- cheap! I use this for cutting fabric for guitar amp coverings, too. It will score thicker plastic sheeting too, so you can snap it off very cleanly.
I've been experimenting with gradually thicker styrene and so far, there's no issue. today I used it to cut out decals. These decals just involved straight line cuts, but it was perfect--one pass, one cut all the way through with no pressure and no slips.