It's good if you only use it to buy things you'd have bought anyway and if you have already checked you can't get something cheaper elsewhere. There is no membership fee now and the promised cashback is nearly always paid without a problem although it often takes a few months.
if you use it carefully you can make quite a bit of money by signing up for things like bingo sites and betting sites- you can get £20 cashback if you bet £20. I used to do roulette and put £10 on red and £10 on black or something like that so I couldn't lose but still got the cashback. But walk away from these bingo/gambling sites once you've met the requirements
No catch. I signed up a couple of years back. I dont use it very often but have had over 600 quid back. You register and then look for what you want on there - eg car insurance and see what offers they have. You then access the insurance site through quidco so they can track what you did and then in time refund whatever the amount is they are refunding. I have also used it for hotel room bookings (12% or whatever). You sometimes have to be very patient for the pay out (longest was about 9 months). Well worth it though as it is money back on what you spend.
There's no real catch but only use it if you are buying the item anyway as some cashbacks are only worth a couple of quid & even then you have to wait months.
But if you are buying anyway you don't have anything to lose as it does mount up.
If you're buying big stuff especially, I got £65 back on a sofa so that swelled the balance.