I stopped in this hotel at the start of October with my partner and my parents also had a room at the hotel at the same time.
The location is just brilliant: right in the thick of it but just off the main street enough to be quiet. Both our rooms were really spacious and the bathrooms were lovely and the showers nice and powerful. The views from our 6th floor balcony were brilliant and the views from the rooftop restaurant where you have your breakfast were just breathtaking.
The food choice at breakfast was brilliant and all really tasty - from traditional cooked breakfast items through to croissants and fruit and yoghurt. We found the staff at breakfast to be really nice if you give them a smile (and a few words of spanish go down well too) and they soon got to know us and were really friendly and helpful every morning. We never had a problem getting a table for breakfast and never had to queue.
We read a lot of reviews of the three main better class hotels in Gibraltar (this, the Rock and the Caletta) before choosing this hotel and we're glad we did. I didn't see in any rooms at the Caleta but we walked through it one day and found it to have a very odd atmosphere and the outside of the hotel and the terraces seemed quite run down. The Rock looked nice but we found a good deal at the O'Callaghan that meant it was half the price of the Rock and I can't image the Rock could be twice as good - plus it is a bit more out of it location-wise.
All in all, this is a good hotel. Just don't have a beer in the hotel bar as it's not great! Stick to spirits or head to one of the many bars in the vicinity. The Morroccan restaurant opposite the hotel is very good too and is recommended.