My daughter just booked a balcony guarantee on Pricess and was upgraded to a much nicer cabin on the Lido deck where she is happy to be. As Dave said, you are guaranteed to get at least the same grade as you booked, but could well be upgraded, higher deck, etc. If you are prone to seasickness you want a cabin midship with a lower deck. If this is not an issue, try the guarantee. Cruise Critic is a wonderful site to learn all you ever wanted to know about cruising.