Only ask for a guarantee cabin in the lowest category cabin that you would be happy with. There is no "guarantee" of an upgrade with a guarantee. If you do get an upgrade then it's icing on the cake but don't be overly disappointed if it doesn't happen.
If you have your heart set on an outside don't book an inside guarantee and if you want a balcony don't book an outside guarantee.
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Also, you have to be happy with you cabin being anywhere on the ship (ie below the kitchen, below the lounge, below the pool, etc.). When booking a guaranty cabin the cruise line can literally put you anywhere on the ship in that cabin category or higher. I've had very good luck with upgrades on my last two cruises by booking a guaranty. On the first I was upgraded from an inside cabin to an oceanview. On the second I was upgraded from a 4a (inside on the Riviera deck) to a 4d (inside on the Empress deck). I was happy with both rooms.
Celebrity Century 1/09
Carnival Destiny 10/07
Carnival Destiny 9/06
Monarch of the Seas 3/06
Carnival Inspiration 8/05
Carnival Miracle 4/05
Carnival Inspiration 4/04
Sovereign of the Seas 4/03
It's all a matter of personal opinion, but we'd be hesitant to book a guaranty. A higher category may not necessarily be a better cabin. As one of the other posters mentioned, you could potentially end up in a higher category, but in a less desirable location (under the pool deck or above a lounge that plays music well into the night, for example). Just be aware of that going in. If the difference in price is worth that (and in many cases it is) , go for it.
Normally we book a guarantee but this time we chose the cabin. Why? The last couple times we hoofed it from one end of the Conquest to the other, 9 times a day! Our favorite spot is by the pool (aft) but they put us in a cabin at the bow. So, this time we chose a cabin that's real close to the pool but not directly under it. This will save us tons of walking and we won't get the noise from the dragging chairs.
Hello! Just booked room 8285 on the verandah deck, catagory 8C. It has a room on top and room on bottom, but is two rooms down from the lido deck above. Is this a good location? Will I hear dragging chairs?
Going on #9! Serenade of the Seas 12-08!