I have. I’m yet to use it for my trip though (this month). But the voucher that you get is the one that you can take up directly to the park gate to scan. I found that it is cheaper than Ares which is where I used to get my tickets before. You also get points when you complete an activity/use the voucher which you can use to save on another purchase. So extra savings there.
I’ve used Klook without any issues when I went to Seoul, so here’s to hoping my experience would be the same.