Ok here are some first impressions:
Twitchy throttle: Holy Cow! I expected it and I was still surprised by it. Waaaaaaay too sensitive. Worse, the gas pedal is at a weird location and/or weird angle, I haven't decided which. After only a couple minutes of riding it, my right foot and ankle were getting cramped from holding it just right to maintain control of it. The only time it didn't cramp was while driving at speed down a paved road when I had the pedal smashed to the floor.
It's impossible to go fast in a RTV-900; it's not impossible to go slow in the -XG850, but it is difficult. The dang thing just wants to get up and go!
My RTV-900 was slightly better at going up steep hills, although it was painfully slow. The -XG850 had slightly more difficulty, but only because of lack of traction or perhaps too much power to the rear wheels causing them spin out or perhaps the weight difference (the -900 being about 200 lbs heavier). Lack of power is not the problem. With traction, it not only goes up steep hills with authority, it does so with enthusiasm. No more of the -900 propensity for the painfully slow "willImakeit?willImakeit?willImakeit?willImakeit?willImakeit? - Yea I made it!" crap.
Kubota no longer Line-Xs their RTVs. According to the dealer, Line-X can't get some material so they are way behind. I e-mailed Line-X several weeks ago about doing my XG850, never heard back from them.
Speed: Has it in spades, especially in contrast to the -900. It's still slow vs the true sports UTVs like a RZR, but for my purposes, it's plenty fast enough. It's just the opposite of the -900.
I haven't towed or hauled anything yet, so I can't comment on that yet, but I foresee no problems, the -XG has plenty of power.
My -XG came with a number of plugs available for power; for extra lights for example. The problem is that they all appear to be Kubota-specific. Anyone know where I can buy plugs to fit them - other than Kubota?
The rear tail gate rattle is just as annoying in the -XG as it is in the -900. A couple pieces of 3/8" old air hose installed on the tailgate latches fixed a lot of this. My thanks to someone here for that suggestion.
The throttle is the only real complaint. To me, this is clearly a safety issue. No telling how many people have had accidents or hurt themselves because of that throttle. Kubota has years to fix it and they've done nothing. It's inexcusable IMO. NTT should step up and provide a decent DIYer and effective fix for this.
