I have one and they are nice, though I always considered them a bit of a gadget, rather than a proper tool, as for their purpose, supposedly for people who bake a lot, they should have come with a fixed blade. I cannot imagine people going around with one in their pocket, just in case they need to slice pastry when they are away from home..A fixed blade will also be easier to clean.
Not a baker myself but is there a huge advantage to the serrations when scoring the dough ?