Would I book a guaranteed cabin?

Yes I certainly would. At the moment there are a lot of great deals for cabins on a guaranteed basis, before booking these though you need to know exactly what you may get.

Look at the cabin grade - the cruise line has to give you this grade of cabin or above. Be careful though they could allocate you a higher cabin grade, which may be on a lower deck, or may be an obstructed view. ie - You may have a guaranteed outside cabin, but be allocated an obstructed view balcony. Check that you would be happy with any of the cabin grades that you could be allocated.

With a guaranteed cabin you cannot choose your bedding configuration - this isn't normally a problem as the cabin steward can always remake the beds however on some cruise lines (Ocean Village and Fred Olsen) you may be allocated a cabin with fixed twin beds. So if you're sleeping arrangements are important then always check with your agent first.

Finally, MSC when giving a guaranteed cabin, do not guarantee that you can stay in the same cabin for the full duration of the cruise. If the cruise is busy there is a very small chance that you will be asked to swap cabins during the cruise.

The plus side of a guarantee cabin is the price, you get the same service on the cruise, everything else is included and you get a great price. Cunard are currently offering a great guaranteed deal on Queen Mary 2 -14June. A balcony cabin on a guaranteed basis is £1199pp, whereas to book a balcony knowing your cabin number you'll pay £2279pp. To save £1080pp I would definately risk an obstructed view.