Worth it if their quote is the cheapest, but don't rely on the cashback
Fortunately I've not had any need to call on the services of Hastings in the year and a bit I've now been a policy holder, so other than a simple 20 minute phone call at renewal time this year to see if that really was the best offer they could come up with (spoiler: it wasn't, all I said was that I'd found some other quotes a bit cheaper, and they managed to knock around 20% off their quote without any change in cover, excess etc.), I've not had any reason to contact them.
Last year they did come up with the most competitive quote which was the main reason I opted to go with them despite having not had any prior experience with them. However, the prospect of £55 cashback (as was being advertised back then for a Premier policy) as well didn't hurt matters - that certainly helped dissuade any concerns I had about insuring with an unknown rather than staying with one of the other providers I knew.
Having done everything by the book (so I thought), I was a bit annoyed to see the initial cashback claim declined, but having occasionally seen this happen before due to a glitch in the tracking, I submitted a claim and waited for the outcome of that. Meanwhile, I'd already had confirmed cashback from the insurance policy I'd taken out on the same day from another provider for my wifes car, so I know there isn't anything fundamentally wrong with how my PC is setup that would prevent cashback being tracked/claimed successfully.
Annoyance turned to anger when this claim was also declined. Partly because £55 isn't a small sum of money to lose out on through no fault of my own, partly because the retailer/Quidco don't provide any specific reason why the claim was declined, and partly because I felt like I'd been lured into buying a policy with Hastings at least in part because of the cashback being offered, without which I possibly would have taken my custom elsewhere.
As the final decision came around not long before I was due to renew my policy, I was seriously considering ditching Hastings even before I saw what their renewal quote was, and when it came in slightly above the other quotes I was getting then my call to them was originally planned to be to cancel the renewal - had they not instead come back with such a significant reduction in the quote then they'd have definitely lost me as a customer.
So whilst their prices are pretty competitive, it's probably best to judge them ONLY on their quote without taking any cashback into account - if you do end up getting anything back from them, consider it a bonus. Not what I've come to expect from the retailers I've done business with via Quidco over the last 7 years...