There are plenty reviews about Cunard's Queen Mary 2 (QM2) so this review is only to add a few points for 'new cruisers'. This is a beautiful, stately ship that's not for the hyper active family but has plenty to keep most people occupied especially the more cerebral.
I booked one of the cheapest cabins and the only regret was that with a window you could gauge what time of day it was but the money I saved paid for extras on board/ shore. The cabin was good, sailing very smooth (didn't know you were actually moving sometimes), staff excellent, food very good, drinks about same price as British pub prices except wine/ champagne which were more, spa treatments way too expensive.
I worried about being 'smart enough' and having enough evening wear for formal nights. Having been in the WRAF we had to wear full length ball gowns on such occasions so that's what I expected- don't worry, you can dress up as much as you want or not at all. People dressed down in the alternative restaurants and some ladies wore trousers or cocktail dresses on formal nights in the Britannia Restaurant.
Wouldn't book Princess or Queen's Grill as you're stuck away in your own restaurants and near top of ship which is prone to much more movement, not good for bad travellers. Mind you some people like being away from the hoi polloi and looking down on us!
Finally, I felt that this ship would be best enjoyed as a couple as the sail away parties and dancing are more romantic and fun when shared with your loved one!