Although in concept I think these vouchers are a great idea, I often find myself either spending extra money on lunch or buying more food than I originally planned just to use the entire ticket (because the majority of places don't give change back). Where I work, there aren't many low-priced lunch options that will allow me to eat a good and healthy meal under 8 euros. It's either the local boulangerie where I find myself purchasing le menu (sandwich + drink + dessert) to get my full voucher value or a nearby café where I end up having to spend around 5 or so euros extra just to eat a salad.
In an effort to spend less money and eat more balanced meals, I've recently begun bringing my lunch to work. So what do I do when I've acquired 4 or more unused tickets? I take my husband out for dinner. Problem solved.