Pros: Keeps milk cold
I've used this bag for a year to carry bottles to work and keep breastmilk cold after I pump at work. It is a little bit big for my purposes, because I just don't get two full bottles of milk a day, but not so big that it bothers me.
Even with only one filled bottle inside, it's like putting the bottle in the refrigerator. When I get home nine hours or more after taking the bag out of the freezer, the bottle inside is nice and cold to the touch. It's also really easy to stick it in the freezer when I'm not using it. No separate freezer bags or other parts to worry about.