Lidl app is totally worth it if you shop there frequently - I hit the £200 mark most months as it's my closest supermarket, so I get the £2 off after £100 spend and the £10 off at £200. So everything is basically 6% cheaper, more if you use the other vouchers.
The weekly vouchers are sometimes useful, sometimes not - but usually for things people do actually buy - i.e. this week there was one for their cheese range, one week it was for anything in their cooked meat range, another for anything in their pastra range.
Sometimes you get a voucher for a free doughnut or similar, always a bonus!
There are also scratchcards/wheel spins after every shop, which can give you specific vouchers or e.g. 50p off your next shop.
The app also stores your receipts, so no more searching for paper ones when you have middle of Lidl regret, or it inevitably breaks...
Never had a problem with the vouchers or app. My only tip would be to load the app and the card up as soon as you've unloaded your shopping onto the belt, as it can take a few seconds to load and holp up the queue otherwise.