I have the Martelli cutters but don't care for them, they just "feel wrong" in my hand. I don't like the Dritz because it's hard for me to get the blade in the correct position. It's somehow related to the way it automatically closes.
I'm okay with the Fiskars but my preference is definitely the Olfa. The safety issue of self-closing versus manual closing is one of training and habit and I long ago made it a habit to close my cutter when I put it down.