I've been encountering more and more people on cruise ships who have booked a cabin guarantee, with many not having any real idea of what it is they've booked.
There's a number of different ways to book a cabin guarantee.
Run Of the Ship
(ROS) is often referred to as "best available cabin". To the uninitiated this can be somewhat confusing. Many people believe, or are led to believe by hopefully misinformed travel agents, that by booking this way they are going to get the best cabin on the ship that wasn't booked prior. They come away with visions of paying for the lowest cabin category cabin, and coming away with the owner's suite.
While I'm sure there have been the odd occasions this has been exactly what happened. It's not the norm, and certainly not even likely.
A number of people I've spoken with on cruise ships lately have been told by their agents that booking ROS would guarantee them an upgrade. This is simply not true. It's unfortunate because people are disappointed after walking into their inside cabins when in a roundabout way they've been led to believe they're going to end up with an outside cabin, or even one with a private balcony.
I suspect all of these are contributing factors, but in any case you're spinning the wheel, gambling that your name is going to pop to the top of the upgrade list.
There is no guarantee to this guarantee.
There's certainly nothing wrong with booking this way. Just be aware that there is no sure upgrade waiting for you. Prepare to be happy with whatever cabin you end up, after-all it means you are on a cruise.
Minimum Category Guarantee
If location within a category is of no concern to you, it's not a bad way to book.
In my opinion knowing what to expect is of utmost importance. If you understand the gamble involved in booking via the "guarantee" it can be an effective way to save money. I just feel bad for those I've met recently who have been misinformed.