I've got 5 acres with a lot of trees and it is pretty bumpy. I started out with a Kubota ZG20 last season, but it's cut just wasn't up to par, and frankly, it didn't really speed up the cutting at all. I ended up trading up to the ZD326p this season and wow! Night and day difference. This model has a suspension seat on it, and the hydraulic lift for the deck and the easy access maintenance are what really sold me on it. Much quieter, more efficient, and it is basically commercial grade. I threw some gator blades on it, and I cut my whole yard last weekend in less than 3 hours. Used to take about 8 hours with the old MTD garden tractor.
When you are deciding on one, try to get the heaviest duty one you can afford, and make sure it has a reliable engine in it. FYI, my payments are about $180/mo on the ZD326p.