I owned a 456 GT 6 speed for about a year. On the positive side it is a very comfortable car to drive great distances in. I drove mine from one end of Europe to the other and did not need an appointment with the chiropraticer afterwards. Engine is terrific; it is fast rather than quick due to the weight of the car. At 100 mph the 456 GT just sits on the highway eating up the miles. 2 adults and 2 kids fit comfortably and it has enough luggage room for a couple of decent size bags. You can even jam 2 adults in the back for short trips. For a 12 cylinder Ferrari, they are reasonable to buy. IMHO, definitely one of Pininfarinas best designs.
On the negative, I thought that they were full depreciated but turned out to be wrong. The window seals are crap and wind noise at high speed is just part of the cars charm. I understand that they more or less fixed this on the 456 M GT. I bought a car with a full service history that had been well cared for by both the prior owners. Despite this I still ended up spending about $5000 in maintenance which included a clutch replacement, new horn, minor electrical problem rectifications, seat motor repairs, etc. All in, by far the most expensive Ferrari I have owned.
In the end however the decision to sell was as much based on the personal preference as it was cost. To drive, the 456 GT does a lot of things really well. Probably too well for my taste. It is the only Ferrari I have owned where I always had the stereo on vs the others where I find myself listening to the engine etc. Net net, we did not bond. I had similar experience with a 911 (993).
If you do decide to buy a 456 GT, go through the car with a microscope yourself first. Make sure everything electrical works not only when the car is sitting on the driveway, but also when pushing it hard on the road. Wiring on these can be an issue and connections that seem OK when sitting, come loose the first bump you hit in the road. The rear shocks are another weak point and will need replacing at some point in the 1st decade. Make sure this has been done as it is a $$$$ job. The clutches are fairly strong but when they start to go, the end is quick. Make sure it is in good condition with a lot of life left. The engine mounts and steering rack also degrade over time. One telling sign is if the nose dives when you hit the brakes. Make sure the stereo works properly as they are near impossible to replace. They are no longer made. After you have done all this, then I would get a Ferrari trained mechanic you trust to do a full Pre Purchase Inspection. The cost of fixing anything on a 456 GT is more than the PPI cost.