This pump is awesome!!! I started using it when my son was 2 weeks old and I was able to get a freezer supply of over 150 ounces of breast milk by the time my son was 4 months old. Great for working mom's because the black shoulder bag is discrete. It allows you to pump either single or double.
If you have any trouble with the pump the Medela help line and customer service is great.
I never used to battery pack, but I highly recommend getting the vehicle adapter so that you can use the pump while on long car rides. I traveled with my family on a 5 hour trip when my son was 6 weeks old and I wouldn't have been able to make it without pumping.
Also, I use the little bag for carrying my son's food now that he's started solids. It's the perfect size.
Before you buy any pump, check with your health insurance to see if they cover the cost. Many do and if they don't, this pump is still worth every penny.