I would never book a cruise unless I knew exactly what cabin I had. Do you save that much money doing a guarantee, it just seems like too much of a gamble to me to risk the change of getting a bad cabin. And to me so just save a few hundred dollars does not seem worth the worry.
Frankly I am not that picky on cabin location. (Have a balcony guarantee right now for October, and it did save about $100 per person). I've also on the short 4-nighters booked one of those bunkbed room guanratees on Carnival and have always been upgraded to a regular inside room, which saved about $40 per person (or, in translation, the cost of tips for each person)
To a large extent, it depends entirely on the ship in question. Also, if you have no special needs on board (such as being near an elevator) and book a guarantee in the minimum cabin category in which YOU will be comfortable then it can be a good way to go. You won't know your cabin assignment generally until a week or so before you sail. If this doesn't sit well with you (and that seems to be the case), then a guarantee probably is not for you. But don't put up a post when you return wondering why the people in the cabin next to yours paid considerably less to be there than you did. One of the things you are paying extra for is certainty.
When money is no issue, we pick the cabin. This year, bad economy, a layoff, money became a big enough issue that to cruise we had to pay the smallest price, which was a guarantee! Doesn't matter where I sleep -- matters a great deal that we get to cruise!
I have two guarantees booked. In both cases, they're the minimum I want and, looking at the deck plans, there's nowhere in that category I would mind being. I sleep like a rock, so I don't mind noise, vibration, being in the back of the ship or the front of the ship.
It didn't save money in either case, but it does provide the slight chance for upgrade.
It's a total crapshoot. The first time that I did it, I got upgraded, if you can believe it, to a balcony cabin. The second time I did it, I got cabin XX01, at the front of the ship. Not a good experience.