We took a transatlantic crossing from Southampton to New York. We travelled in a P1 suite on the port side (specifically booked so we would see the Statue of Liberty from our balcony).
The ship is lovely. It's classic ocean liner and if you're looking for this kind of experience then this is the ship for you. The transatlantic crowd is quite cosmopolitan and certainly dresses for the occasion. The formal nights are quite a sight and you will see many spectacular dresses.
If space and luxury are what you're after then you will enjoy a Princess Grill suite. It has a walk in wardrobe and feels extremely spacious with lots of storage space. On long cruises I would feel very comfortable in a cabin of this size - it's very generous and well appointed. Fresh fruit is changed daily and the bed was very comfortable. We slept very well.
The staff on the Queen Mary are amazing. Every staff member went out of their way to say hello to us and make us feel special. Our cabin steward kept our room immaculate and the Princess Grill restaurant staff were incredible.
The food in the Princess Grill is fantastic. We enjoyed all the dishes we ordered (especially the lobster which our waiter kindly arranged for me to have three on the one plate when I was concerned that one might not be enough. If you dine in the Princess Grill you will feel like royalty, as we did at every meal.
We loved this cruise and we really appreciated the staff making us feel so special.