I've got a Deere Z920 (2007 model, I think) I bought used for $7500...4 years back. It had about 200 hours or so...now around 400 hours. I mow 2.5 acres of lawn (my lawn) and also use it to tow a lawn roller (48" wide) and a core aerator (also 48"). It's been a great mower...fast. No suspension seat and it's pretty rough, even on a fairly smooth lawn. I should add a suspension seat, but I keep thinking about trading for a mower with a bit more horsepower.
My neighbor also runs a Deere ZTR...a 930. He mows his field (mostly lawn now, but didn't start out that way) which has got to be a real challenge even for a commercial mower (both the 920 and 930 are commercial mowers).
So far, both mowers run great. The weak point on the Deere ZTRs is the PTO switch. I've replaced mine twice...neighbor, about the same.
Anyway, a solid recommendation for the Deere commercial grade mowers...