I used to pump using Avent Isis, and did have the back log of milk that Nicole described, but then learned that it was because I didn't hold the pump right. Actually you have to hold the pump so that the bottle is in a complete perpendicular position with the floor... otherwise, it might build up a back log of milk, or even leak milk. Other thing that drove me crazy was the squeak sound... From some reason, the pumps would sometime make some annoying squeak sound each time I compress the handle.
If I am going to buy a new pump again, i might try the Medela Harmony instead.