I sailed on the Queen Mary 2 and the Celebrity Millennium. In the big restaurants I found the food comparable, and thought it was just great.
I like the breakfast buffet on the QM2 as they served black pudding - if you aware of that traditonal English fare. Alas, on both ships, the oatmeal could have been much thicker... if the spoon don't stand up then...
Other than that... no compliants at all.
As for service on the vessels... it was basically equal, though socially I much preferred the European waiting staff on the Queen Mary 2. Very friendly, took time to carefully suggest and describe meals, and offered interesting short conversations, light amusing anecdotes, and had a good sense of humour. Our head waiter - Erwin - was almost like one of us and we often felt like asking him to pull up a chair and have a coffee.
The Todd English restaurant on the QM2 served the best dinner I have ever had in my life. The coffee, the dessert, everything vaulted expectation. The main course - steak - was so out of this world I had tears in my eyes. Even my wife was in heaven. We had never tasted food like that in our life.
We have cruised with Celebrity many times (Mercury, Galaxy, Century) and with Cunard (Queen Mary 2) only once. We found the food to be comparable, with Celebrity holding a slight edge on service in the formal dining rooms and the pastries overall. Cunard's casual dining, however, was excellent. Overall, because of the grandeur and history, Cunard is superior.