I have a GX335 with the 54" deck, and it's been an excellent machine. I mow about 5 acres during the summer, and haul firewood with the 15 cart during the winter. The 20-hp Kawasaki has no trouble cutting through really high, thick grass. And during the winter, firewood is no issue either; I haul mine up a pretty steep hill. I also loaded the cart down one day with 34 bags of topsoil (40 lb. each, total 1360 lb.), and it didn't have much problem with it. The deck comes off/goes on in about 5 minutes, and the power steering will completely spoil you.
Overall, I'd recommend one. I paid about $6300 for mine new in April 2003. I never checked the Kubota line for their prices, but I'm sure their machines are just as good as Deere, although I really don't like their treadle-pedal setup on their hydro. The twin pedal is far easier to manage (imo). If you can get one for less than $6k, that would be the way to go.
Good luck with your decision.