We have used booking.com on many occasions with no issues but will not be using this site again after our stay at Gemelos 22, Benidorm in April 2015. We had issues getting the keys, first we were sent to a near by hotel only then to be sent to an office on the beach front. Once there we could not collect the keys without paying a 50 euro cleaning fee which pays to clean the apartment on departure! This was not made clear when booking the apartment as otherwise I would have booked through a different site which was cheaper.
When we got back home I tried to complain to booking.com who after a few emails offered to refund half the cleaning fee. Although they felt the cleaning fee was clearly stated (however it is not in what their own site calls 'the fine print')they did admit that with the cleaning fee added on to the total their website did not meet their best price guarantee. For speed I agreed to the refund of 25 euros only to then have not one but five emails ignored when I tried to arrange this refund.
In the end I had to contact Trading Standards who advised my to get in touch again and if I had no response to pass on all correspondence between myself and booking.com on to themselves for them to investigate. So sent another email and then I was offered a refund of the full 50 euros cleaning fee. This was dealt with in a matter of days once Trading Standards were involved and the refund credited back to my card, a big difference to the weeks and weeks where my attempts to contact them was ignored.