You can use it anywhere that accepts Mastercard. Looking at my transaction history I used it in the Contemporary Resort gift shop, Apple Store, Wal-Mart, one of the pin carts in Animal Kingdom, World of Disney and one other shop in Disney.
You do get a PIN with it but I was never asked for it and on occasion I didn't even have to sign, which is part of the Disney system of keeping queues down.
I have $7.69 sitting on mine at the moment and that will stay there forever and a day or until I spend it, whichever comes first.

There is an expiry date on the card but if you have a balance left they will transfer it over to a new card for you less a replacement fee which, looking at the website, I assume is the same as the fee for replacing a lost or stolen card - £5.99/$12.00. You could also request a refund of the balance left on the card but they do charge a "cashout fee" of £10 for that.
If the card is lost or stolen you must report it to them, they will block the card and issue a new one. Again the joy of it being prepaid is that your maximum loss will be whatever the balance on the card is rather than losing a debit card where the thief can drain your account, or a credit card where they can run up to your credit limit, unless the bank blocks them in either case.
I don't think there is anything else you need to know. I treated mine as kind of a savings account before I went. I had a couple of months where (unusually for me) I had some spare money at the end of the month (rather than spare month at the end of my money

) so I just put it on my FairFX card and it paid for my holiday souvenirs, rather than wasting it at home or running up a credit card debt that I would have been paying off for ages.