Mine arrived this morning 
I paid the last payment early online.
You select the vouchers/gift cards you want and they tell you how much that works out to each week or month depending on how often you want to pay. I've already selected mine for next year.
This time I have selected theatre vouchers aa well so I can take my DC to watch a show next Christmas. Also an M&S giftcard so I can buy lots of lovely food. Bit disappointed that you can't buy Amazon vouchers separately anymore. They are combined with the love2shop vouchers which is annoying. I like the love2shop vouchers but Amazon is convenient. Also selected a Smyths voucher to use when they have the 20% off weekend.
If you change your mind about which vouchers you want, you can always phone them and they will change it over. So although I've selected those vouchers I know I can change them next year if my situation/plans change. I changed mine a few months ago and it couldnt have been easier.
I prefer Parks to a savings account as it means I can't dip into it throughout the year and end up with nothing come Christmas.
The only word of caution is that if you select gift vouchers make sure you spend the full amount because you dont get change with them, you just lose the unspent amount. I choose gift cards as they dont have this issue. If you spend £10 of a £20 giftcard then you still have £10 left on your giftcard.
I have also ordered a prepaid Mastercard for the first time. This is the only one that carries a fee; 10%. So a £100 card will cost you £110 but i think its well worth it because you spend it anywhere that accepts Mastercard.