Interesting to read other recent reviews - our experiences were very similar. It wasn't a bad cruise and there were highlights - we enjoyed our cabin and the housekeeper was generally good - but otherwise we left thinking that we wouldn't rush back to Cunard.
Service was our biggest gripe- Britannia restaurant was very hit and miss - one head waiter actually told me off loudly for sitting at a table that we had been shown to by another waiter. Definitely not customer-focussed and I went to see him later and said that he was rude, discourteous and embarrassed us in front of other guests ("but the other waiter made a mistake....and he told me..." etc (seriously - who blames colleagues or the company to excuse poor service in a 5 star set up?) Often waiters were indifferent, surly and rude but occasionally pleasant and helpful. (And Cunard expect you to pay service charges for everything - even when the service was just plain bad). One night we waited an hour and 15 for our main course. Delays in getting bread and wine were also a feature - a bit of a macho culture going on with some of these guys.
Food was ok but definitely not 5 star - it wasn't bad as such but God knows how you'd do a long cruise - I'd lost the will to live after 7 days. Desserts in particular looked great but were often synthetic tasting.