I would think by booking this way if you don't have any preferances as to what type of cabin you get or where it will be, then this is for you. On the other hand, you could get anything. I think the safest bet is to book a guarantee cabin catagory, where you just book the lowest catagory you'd be happy with and if you do get an upgrade, thats would be icing on the cake.
For me, I just have to have a balcony, but will still book a guarantee on a balcony catagory, no way will I be disappointed that way. I've done 2 so far like that and at this point have not received an upgrade, but its a gamble any way you do it.