Look at Ferris ZTR's. You will be impressed. The 4 wheel steering JD is a fad tractor. Remember when GM had the 4 wheel steering trucks, they lasted about 1 year for a reason. Maintenance pigs.
Chirs
This statement about 4 wheeling steering is just plain wrong. I have a JD425 AWS. It's 15 years old, 850 hours. Only maintenance on the AWS is to grease the 3 zerks on the rear steering system. Works superbly and is actually quite simple: 2 u-joints and a center pivot arm. I've spent a grand total of $75 on repairs on this tractor, and that was for a new ignition module that has nothing to do with the steering. JD has been making them for more than 15 years. That's some long-running fad!
What it really boils down to is a ZTR is faster if your ground is smooth enough to take advantage of it, and you don't have steep terrain to deal with. A ZTR turns tighter. But in practice not by much for most users chores that involve trimming around flower beds, trees, and posts. The X749 gives you a tractor that can do other things besides mow: plow, tow, dig, lift, log splitter, etc. The ZTR is hard pressed to do these things. So if you have tons of smooth grass to cut and that's all you want to do, then a ZTR is a good fit. But, otherwise the tractor is more versatile. To me, the real issues with JD AWS tractors are that you cannot put the JD loader on tractors that have it, and it lowers your ground clearance significantly. But, the x749 AWS is a very fast mower, and still can do other things too. I mow 2 acres with a 54" deck, with 50 or more obstacles to mow around in about 1.5 hours. And I seldom need to trim afterward, just a small amount about once per month in tight corners where no deck mower can ever get at.
I also have a 2002 GMC all-wheel steer. Works just fine. GM made them from 2001 - 2005, I believe. They quit installing them because of cost vs. demand. Few could justify the $3000 price tag. But, I love it for parking and for towing: unbelievable turning radius for a big truck. GM did have some troubles with rear differentials married to the 4ws rear ends. But these problems really weren't related to the 4ws mechanisms.