Reviving an old thread, but still relevant. I purchased a Kubota Z421 with the Kawasaki 24HP engine and the 54" deck. I have used it for a year and a half now, commercially.
The carb had to be replaced under warranty almost immediately, but otherwise the engine has been flawless. The FS engine is one step below the FX engine, which is the best engine on the market for commercial zero turn mowers. The primary difference between an FS vs an FX is that the FX has a superior carb and a better air filtration system. Engine internals are nearly identical, and I'd expect at least a couple of thousand hours of engine operation before rebuild / replacement. I've never missed the nicer FX carb, but the air filter system is definitely better on the FX...mushroom style. It is smaller and lower on the FS, and in dusty conditions can clog quicker.
The mower itself is overall pretty decent. Kubota has moved away from using any grease points on the mower deck / spindles / pulleys / casters. This is fairly common now, as greasepoints only extend life if they are greased at the proper intervals. Too much grease pushes out the bearing seals; too little causes loss of lube and introduction of dirt. The cut quality is good, and the Hydro-Gear ZT3600 transmission is commercial quality. Hydro-Gear integrates the pumps and drive motors into one unit that is less susceptible to leaking than discrete units. I find this mower to be faster than its 10mph speed rating implies, and it is very responsive. Mine doesn't have the wider rear tires, but the low profile tires and rims seem to grip side hills considerably better than my larger tire zero turn from another brand.
The deck lift is smooth, but the pedal is high and awkward to use with a tall operator given the position of the lap bars. The foot parking brake is easy to use. The cutting height knob, similar to the deck on a BX or B series tractor, suffers from the same problem here as it does on the tractor application: dirt gets in the mechanism and gums up the action, making it hard to adjust the height once you've been at the same height for awhile.
The fuel tank does not reside inside the fenders as is common, but instead is under the seat. It holds 7+ gallons which is more than the usual 5 gallons for mowers without dual tanks. There is a sight gauge on the tank, but you can only see it when you are not sitting in the operating position. Because the tank is under the seat, the engine is mounted off-center in the rear cage, leaving room for a fully-exposed battery off to the left. The fuel tank also covers easy access to the hydro fans.
I do like how the hour meter counts any time the engine is running (some mowers only start the hour meter when the blades are turning). I also like the provided 12v outlet and nearby slot for inserting a phone, but I wouldn't want my phone in such a dirty location.
The seat is nice-looking and comfortable, but mine seems to be getting loose on one side.
At 110 hours, my HST drive belt just shredded and the tensioner spring fell out of the bottom. That's not encouraging.
Overall, this seems to be a very good mower, and a good value.