J.C.
I pumped with a medela with both of my kids, and for child #2 I was also able to borrow a whisper wear from a friend, and used that sometimes. I also read a lot about it online from other mothers. It seems to really depend - some women got a lot of milk using it, others needed harder suction.
I found that it was enough pump to relieve pressure and get some milk, but I didn't get the quantity of milk I did with the Medela. I found it really useful when I had to be offsite from my office and didn't have a private place to pump, or handy and discreet while driving.
I had a hard time finding the Whisper Wear parts at the time though (that was 2 yrs ago) and the people at Babies R Us told me they thought the company might be going out of business, so investigate to make sure you can get parts, or possibly borrow/buy used, rather than making a big investment.