We have used it each trip we have taken over the last few years. You generally have to buy a package through AAA to get it - you can't purchase the pass. That being said, we bought just our tickets one year through AAA and got the pass. Another year, we just booked the hotel and got the pass. When you're booking, just ask if it will be included. It doesn't entitle you to free parking - if you're not staying onsite, you'll have to pay for parking.
The pass is wonderful for EPCOT and Animal Kingdom - you park right up front. At MGM and the TTC for Magic Kingdom, we found the walk to be longer and the pass not as beneficial. But it is still walkable, unlike many other spots in those parking lots! You just put the pass on the dash of your car and you'll be all set!