SZ gty is a great way to book with HAL always. When they run their gty promos (the best are called SWAT or Flash promotion, the flash promos are also called 3 day or 5 day sales) they always offer them in the lowest available category. They do this even when they have no more cabins in the category you're booking. that means they have to upgrade you if they have no more. There are always far more of these cabins sold during these promos than they have in inventory. Same applies to balcony gty in Cat VH. The vast majority of the time you'll recieve a small upgrade to the next two higher cats. Occasionally they'll leave these open even when all the have left is mega-suites at much higher pricing. Someone is going to get an unbelievable upgrade in that case. I've had two clients get so lucky in the last couple of weeks. It happens a lot more off season.
Unless you have serious motion issues and have to be midship or you just feel you have to know where your cabin is it's a great way to book. You'll always get beeter rates than those who choose their cabin and sometimes end up in a better cabin than those who pay more and choose. Your Mariner status (their loyalty program) does have an impact on who gets the premo move up. If you're a first time cruiser with them don't get your hopes too high.