They are ok, but the implementation in some stores is terrible. The one closest to my home has a scale that measures the weight of the products you scan. Almost every time I use it that scale decalibrates itself or doesn't properly detect that I put the product down, and locks itself until a cashier comes and solves the issue. And it locks itself for other reasons too, like if a card payment fails, you can't retry until the cashier comes and unlocks it. The only advantage is the smaller queue.