The fastpass option allows everyone in your group (up to six people) to receive a FP card that is good for up to six rides in the park where it is redeemed. We went to WDW (2 main park days)and Universal this summer, and we all had two days of tickets left except for DD11, who had one day. We redeemed her voucher for admission and the rest of ours for FP cards.
Due to the FP cards, we rode Toy Story Mania three times even though the regular FPs ran out at 11:15 that morning. We used our park tickets for FPs for ToT and RnR, so we saved about five hours of waiting on a very hot summer day. We were able to avoid the rope drop stampede to TSM and were able to get instant FPs after the regular ones were gone for the day.
At MK, we were able to avoid criss-crossing the park to collect FPs. If we arrived at a ride and didn't want to wait, we used an immediate FP.
We hopped to Epcot one evening and FPs for Soarin' were gone and the wait was 100 minutes. We rode immediately. We went to Test Track and the wait was 80 minutes. We rode immediately and gave a family of four instant FPs for Test Track. They were speechless.
We could have saved our old tickets, but the FP cards allowed us to travel with less stress (my kids get a bit nervous at the TSM rope drop stampede) and less planning (no sending a runner to get FPs across the park). We go to WDW almost every year, and the FP cards allowed us to travel in a way we likely won't be able to do again. It wasn't worth the cost savings to us to give up the perks in July. But if you are going at a slower, cooler time and don't already have tickets, upgrading the GAD vouchers might be the way to go.
More info on the Fastpass Exchange cards can be found
here.