I have been expressing for the last 8 months. For 5 months I soley expressed but following a bout of food poisoning and milk reduction I supplement with about 40% formula.
If for whatever reason you decide to express rather than breastfeed it is really important that you do not over stimulate your breasts. When you express it is best not to do it until the flow reduces to just a few drops. I did this at the start and ended up with 3 bouts of mastitus, one that resulted in IV antibiotics and 2 days in hospital. Also, I personally found that a handpump was more convenient, easy to control and less harsh on my nipples.
If you produce more than your baby requires it is a good idea to freeze it. This helped me when I had food poisoning.