My little 13 acers are perched on the side of a rocky canyon with a river at the bottom. The only time the RTV900 has been stuck was when I got it out on a recently exposed mud flat and it buried itself up to the axles. Took my 4WD tractor and 50' of chain to get it out. It'll go up really steep stuff, but if the ground is loose the tires will lose traction, and even in 4WD I might not make it to the top. I've also had it up on three or two wheels a time or two side hilling, something I don't like to do but it will handle much steeper slopes than will the tractor.
As others have mentioned, it has plenty of power but doesn't go very fast, so I've turned down offers to go 4wheeling with my friends on their quads or Razrs. It's a work vehicle, it'll haul a yard of gravel, a quarter cord of wood, and most anything needed to set up with a generator at a remote job site. In the spring I bolt a sprayer to the tail gate and run it from the comfort of the driver's seat. Often I'll get out and spot spray with the wand, which has a newly lengthened 30' hose attached. When it comes time to finally get a boat, I'll rig a lifter on the RTV and use it to get to the water. I use it most days to fetch the mail, and once a week to get the garbage can out to the street. Mine has a roof, trailer hitch, and dump bed, and I haven't seen the need for anything else.
About the only thing I've changed on it was to add a snorkel to the air intake to keep dust out of the air filter. A couple PVC plumbing fittings and some radiator hose was all it took, but if I had it to do again I'd route it all the way up over the roof instead of hooking it into the roll bar. Just makes too much noise right above my head with all that air going into the holes I drilled in the 'bar.
Mine's an '05 with over 3000 hours, and so far I've fixed the radiator once (a stick went through it) and replaced the u-joints on the rear axle and forward propeller shaft. I bought it used, and also had to replace one of the steel water fittings where it went into the head. Just turned up a replacement on the lathe and used red Loctite to stick it in. I replaced the fuel filler cap with a newer version from Kubota that goes on and off easier. The previous owner laid down a layer of steel treadplate in the bed, and one of these days I'll reinforce the top rails with some angle iron and give the whole thing a new coat of orange paint (the bed's been an ugly primer gray since I bought it).
If I wanted something to chase around the woods with my friends on their quads, I'd probably get a quad. But for the jobs around here, the RTV has saved me miles of walking and let me tackle projects I never would have without it.