I have a G1800 and love it for its intended purpose - lawn/garden tractor. I probably abuse mine by using it to mow in a rough borrow ditch that is too steep for comfort on my B series Kubota, but it has never broken or let me down in 250+ hours so far. The quality and strength is very high. With reasonable care, I expect the diesel engine will outlast me.
I have two minor complaints; the cooling system isn't overly large (for Texas) and you have to be careful to keep all the screens and the radiator clean but if you do it won't run hot. The second thing is just that the little tractor is only 2WD and the turf tires will spin easily if the ground is wet at all. You'd probably never notice this unless you were mowing in a steep borrow ditch, but I frequently get mine stuck in that situation. Like I said, it's probably not the right tool for that job.
Like Neil said, it does have a mid-PTO to drive the belly mower. The mower goes on and off easily and quickly, is easy to service, and gives a good cut as long as you keep the blades sharp and don't use it for brush hogging.
I don't know what the second-hand market or prices are for these machines - they were very expensive new.
I've never had a Cub Cadet, but if I had the choice of a used Kubota diesel over any other lawn tractor or riding mower for a similar price, I would pick the Kubota every time. In my opinion, Kubota is like the Toyota of tractors - you will pay more up front for them than many other brands, but the quality is tangible and their longevity makes them good value for the money.
