I will also vouch for the JD455 being one of the best garden tractors ever built. We have owned our 455 for over 15 years. We've never had any engine/drivetrain/mechanical problems- only issue encountered is repairing a few flat tires. We own a small farm and probably mow 2-3 acres of rough terrain during an average week in the spring/summer. Over the years we have mowed several of our friends and neighbors lawns/property and they all comment on what beast of a lawnmower it is and want to take a closer look at it. You really can't even compare it to a standard garden tractor.
The 22hp diesel has loads of power/torque. With the 60" deck, it mows extremely quick. I found that replacing the standard blades with a set of gator blades really improved the cutting quality/performance. The large 26x12" rear tires and the differential lock really give the tractor plenty of traction- I can't imagine how much traction you would have on a newer X748 or X749 with 4wd and HDAP tires. The high back driver seat is really comfortable (for a lawnmower), the diesel isn't too loud at full rpm, the glovebox, power steering, and tilt wheel make driving the tractor a pleasant experience for anyone.
In hindsight it would have been nice to install a 3pt and rear pto at the time of purchase. I'm probably going to upgrade to a set of front/rear HDAP tires this coming season and try to custom fabricate a front brush guard.
If you get lucky, you can get a really good deal on a 455. If the price was right, I would feel comfortable buying a nice looking 455 with under 1500 hours. In my opinion, it's not worth the extra $$$ yet to get the newer X500 or X700 series.