No cashback, very good when it works, truly awful when it doesn't
In terms of cashback, they rejected my claim without any reason. Therefore I wouldn't bank on receiving this if you use the Quidco referral.
The service itself is very good when it works. It's easy to register, easy to hire cars and you simply unlock the car you have hired using your credit card, take the keys from the glove box and drive off. The value is good although the invoicing is obscured and therefore charges incurred are difficult to reconcile.
Where it all falls apart is when things go wrong. Their customer service is woeful. Perfectly understandable problems (e.g. the contactless card reader on the car is out of battery or the designated parking space is blocked when you return the vehicle) become a significant hassle as you attempt, often in vain, to get hold of someone in the clubhouse (call centre).
The biggest flaw which is well documented on review sites is their handling of damage to the vehicle. A service like this must be built on trust and if you damage a vehicle you're required to report it. However, if it's damaged after your rental period (e.g. hit whilst parked) but the person after you reports the damage, they will hold you accountable.
Only proceed if you are aware of the risks and it's highly recommend that you insure your excess with a third-party as no matter how experienced or careful a driver you are, it's only a matter of time before Enterprise will lodge a claim against you.