My kids got into the pressed pennies last year and it was great. we loved finding the machines that were tucked away...and contineu to find them at other places we visit (Sea World, Boston Aquarium and got a great Red Sox penny at a rest stop on the Mass Pike!).
I did clean the pennes after reading a lot of posts and it was not easy. And, although I spent a lot of time doing it (all different methods), they didn't come out that great, and honestly, you could see the design better on the new shiny pennies.
So, for this trip, I will be getting a roll of new pennies to put in my M&M tubes with my quarters. Get 2 tubes...one for the money (2 quarter and 1 penny stacked) and the other is for the pressed pennies. That way I don't have to carry the books with me (they will stay at home this trip), and you can keep track of them.
There is a great website that lists all the places and different designs for WDW...
http://www.presscoins.com/
They even have a tracking log so you can mark off which ones you have.
Personally, my kids liked the ones where you can crank it yourself...the autmatic ones were neat, but it was fun watching them wind that handle around...