I have used HolidayAutos many times and in the past they have been fine.
However, on the one occasion when Dollar/Thrifty gave me a car that was not fit for purpose and were rude and aggressive about it, HolidayAutos were nowhere to be found.
Initially, they said that they couldn't deal with it whilst I was away and the rental contract was in place. Then they took months to investigate the matter with Dollar/Thrifty. And finally they said that Dollar/Thrifty rented a perfectly acceptable car - which it wasn't.
I was offered a £50 discount voucher as a gesture, but what I wanted was for them to come back to me in a timely and professional way and to have investigated the matter rather than just ask Dollar/Thrifty about their side of the argument.
In short, use them as they are cheap, but just hope and pray you don't need their customer service.