I use Quidco. They are very safe from my experience. The site just links you to the retailer and you proceed with your purchase. Depending on the retailer your order is tracked and the cashback will show in the Quidco account as pending until the funds are received from the retailer.
No catch really but deduct a £5 admin fee when you first earn over that amount. This covers the next 12 months.
You can withdraw funds directly to your bank account and a small 5p admin fee is deducted from that to cover costs.
I've made £20 cashback on various purchases in the last 6 months...definitely better than nothing!