Used them twice last year, once thru TopCashback, no problem with the rental, picked up a 7 seater Town & Country from Dollar MCO, booked in April collected in August and still waiting cashback, sent them a message yesterday.
2nd time thru Quidco, booked Oct for Oct collection from Dollar TPA, status of cashback says tracked but still not paid. That was a Chevy Traverse.
Only slight problem I had was their lowest price gurentee, I saw a lower price(only by a few £) took screenshot(Netflights) and tried to claim, took them a few days to respond, by this time the netflights price had gone back up so would not pay out.
I would say don't rely on their cashback or lowest price guarentee but no problems with the vouchers or cars.