It reduces your wait in lines at most rides. If the park is busy, and you want to ride as many rides as possible, it's worth it.
Almost have to buy it if you want to get on all the rides. Doesn't work for hagrids motorcycle yet, waited 4 hours for that one. The price changes depending on what day you go so you might want to plan ahead. They're not cheap.
As above.
It is very similar to the fastpass that you can get in Disneyland which allows you to go into a shorter and faster line. The difference between the two is that you have to buy the express pass and the fastpass is free.
Thanks! Your answer is awaiting moderation.