People are saving more because they are combining the deal with other incentives. Simply put, you need to go through the MSN Passport portal, select one of the listed stores and go shopping. During your check-out, you have to select a special button added by Microsoft to recalculate your order for the new lower price prior to finalizing the check-out.
There are other incentives from some of the stores listed @ Passport. Mercata was giving away $100 in credit which you could use to apply towards 20% off of your purchase. Unfortunately, you had to sign up by the end of yesterday (11/17) in order to get in on this deal. However, if you are looking for electronics, 800.Com has some great deals. 10% off of DVD players and large screen TVs. Plus 800.Com has some coupons (ex. $30 off a $150 purchase) which you can find if you look though the other postings in this forum. Combining these 'additional' incentives with the Passport deal is how people are achieving 30% or 40% (or more) off of their purchases.