It's the only way to cross the Atlantic if you have the time, especially eastbound to avoid the jet lag and because you usually have a following wind, which makes walking the decks pleasant, even in September and October.
We always go for an inside cabin, which means we pay only a little more than economy air fare (and much less than business) and get 7 nights and 6 days of first class accommodation and food. Some people say they need an outside cabin or a veranda. We only sleep in the cabin, so we don't care.
QM2 is a superb ship. Except for crossing the Atlantic, we cruise with Holland America. We have previously found the food on the QM2 to not quite match Holland America, but this time (September 2011) it had improved considerably. The Cunard crew are efficient and courteous but mostly not as warm and friendly as the Indonesian and Filipino Holland America crew. And then we have the class system on Cunard that I prefer to do without, even though it is not too intrusive on the QM2. One big difference is that on Cunard, if an officer or other senior crew member sees something that need fixing, such as a dirty table or something dropped on the floor, we have noticed he or she will call over an underling to fix it. On Holland America, they would do it themselves.
That being said, transatlantic on the QM2 is a magnificent experience. The main problem is that there are not enough transatlantic sailings!