I had a Gravely P260z which I traded for an x595 with a 62" mid mount deck. I ran the Gravely over 100 hours, and checked mowing times on the lawns I did regularly. Interestingly my 595 would come within 5-10 minutes of finishing as fast as the P260z would on similar sized lawns, and this fact was remarkably consistent. The other consistency: all the lawns were very bumpy. This clearly slowed the ZTR down and allowed the conventional mower to keep up. The other nice facts in favor of the tractor were that it used less than one-quarter the fuel and was much more comfortable; it had headlights and when I was done, I went and tilled the garden with it. Although I realize we are talking about JD ZTR's here, but there do not seem to be enough design differences in most of the ZTR's to offset their marginal performance on bumpy terrain (except the Ferris, I assume). I could not justify for myself, not being a commercial mower, the sacrifices I made to achieve just a few minutes gain. That said, I found the Gravely to be excellent, and the ZTR concept seems good. I do caution those that ask me, though, to consider an efficient tractor, like the x595, before committing to the ZTR, whatever brand. So far, it's been about a 50/50 split.
John M