These stores are all over Paris so if you're stuck for breakfast or fancy a cheap picnic lunch, there's a decent choice. They sell all sorts of soya milks and breakfast cereals, as well as cakes, breads and fruit and veg. There's a special fridge for ready-made food, including a nice cous with veggies, a lentil and tofu dish (bit salty but tasty), veggie wraps and spreadable tofu with cucumber. I'd advise taking your French dictionary because the staff were useless.
Cons: You'll need to translate ingredients lists, You'll be making up your own meal