When returning back to work it is important for the breastfeeding mother to be able to express milk and store it in the freezer for use at daycare. I have nearly exclusively used Gerber breastmilk storage bags for this purpose. There are graduated marks that measure 6 ounces, but the bags will hold 8-9 ounces. They have a zip top that is easy to close and a place to write the date on the bag. They also have tabs that make them easier to use with bottles that have disposable liner systems.
A problem with these bags is that occasionally they will leak at the seams when thawing out the milk inside. However, I after contacting the company regarding this matter I was sent coupons for two free boxes of storage bags so I rate their customer service highly.
Recently the price of these bags has increased. Because one of the benefits of breastfeeding is saving money on formula, having to purchase these bags at a high price was defeating this purpose for me. I have begun to use regular store-brand ziploc bags to serve my purpose with the same favorable (yet cheaper) result.
If you are looking for a great breastmilk storage bag, Gerber produces good ones. Great ones are more expensive but can be cost prohibitive. Therefore, Gerber reaches a happy middle ground.