Park don't operate the way farepak did, your money is as safe as it is in a building society.
It works for me, you can spend the vouchers in lots of shops, Boots, ELC, BHS, Debenhams, House of Fraser, Argos, Wilkinsons, Waterstones, HMV etc. you can buy M&S vouchers, Burton Group and others as well as Love2Shop.
My money goes out direct debit, I can't touch it and they give it me back in November.
If you pay in £450, you get back £450, plus a bit of commission.
A building society account is fine if you know you're not going to be short one month during the year and dip into it.
Like everything else, it's about finding what works for you.