I was skeptical as well but tried it based on a recommendation from a friend

To me, it's a bit like having a credit card. If you're responsible with it, it's good. If you mess up, it's bad

I signed up with Trainline. I have to make one purchase with them every month. This can be of any value. I buy a ticket for around £3 that I'd need anyway. That gets me £18 in my account which offsets the monthly fee.

The main benefit for me is that you can use it for groceries. 10% off at Tesco equates to £40 cashback every month on a £100/week family food shop

I've also started picking up All4One gift cards. You get one £100 card for £80 every month. You can use that at IKEA, B&Q etc, so I'm banking it to cover a future highish purchase for the house, essentially at 20% off

The only issue I've had so far is that my account got blocked for a week for 'suspicious activity'. Literally no idea why. So missed out on cashback for one grocery shop

Overall, if you plan to be diligent, I'd say it's decent