I'm using them but I'm a stay at home mum so I dont't pump very often; only during emergencies. The milk trays I have freezes milk into 1 oz bars. So after they are frozen through, I remove them from the bars and pop them into a storage bag. I can keep about four bars per bag and that helps me save up some space in my freezer as I can stack the bags up.
I did hear that bags will take up more space PLUS when you want to thaw the milk, you have to thaw the entire bag. With these bars, I can just thaw 1 oz if I want to (to use in intro-ing semi solids) instead of the whole bag.