If you plan to just pump for an occasional time you will be away from your baby, you would not need as good a pump as you would if you plan to pump every day while you are away at work. The occasionally pumping mom can often get by with a manual pump or even hand expression. If she plans to pump a little more frequently, a small electric pump works well. The mom who will be away from her baby while she works, or the mother of a premature infant who wishes to pump her milk for her baby will do best with the larger electric pumps. The larger pumps are available for short or long term rental, or purchase.

The best pumps will automatically release the suction (as opposed to you having to press a button or hold an opening closed), and will cycle frequently. No pump will ever match what a baby does at the breast, but some mimic the mechanics better than others. 

Nancy, PT's webmother - recommends the Medela Pump In Style - especially great for working moms.  It's a double electric pump, easy to use and easy to transport - even comes camouflaged (looks like a large black purse when it's all closed up).  Now known as the Pump In Style '98, the set includes an insulated pouch, coolers & bottles, and now also comes with  a car lighter power adapter and a manual breastpump conversion kit."