It really depends. As
@gelatoni fan mentioned, you can definitely save money if you're fine with their specific meal offerings. The offerings are limited and you're only allowed to use the vouchers at certain restaurants/locations. For example, at one food court location, all of the lunch meals were priced at HK$119 or less, so you would never get full value there. At the Royal Banquet Hall, there were more dinner options, but also some items that did not qualify that we would have liked to try. We did skip other restaurants/food options only because of the vouchers, which we would have loved to stopped for otherwise. And there were a couple of the voucher-accepted food locations that were closed (disappointing!), requiring us to walk much further while already very hungry. If saving money is your primary goal, then the vouchers are fine. We probably wouldn't buy them again, as we want more freedom to eat at any restaurant nearby when we're hungry, and the flexibility to sample anything that looks delectable at the moment. And the difference in food costs probably isn't significant for 1 or 2 days of HKDL.
Regarding use, we bought online and they scanned the codes at the entry gate, and gave us our park tickets and meal voucher tickets at the same time. At the restaurants, they just scanned the meal vouchers at the cashier. Very easy and no issues whatsoever.
Hope you have a great trip!
ETA - Added some photos (hopefully you can see them)
Lunch:
Royal Banquet Hall (dinner):