Snapper is owned by Briggs & Stratton. It shouldn't be too hard to turn up reviews of Snapper ZTRs. The 550Z is still in the residential line of mowers, but it's at the upper end of those, so it has some nice features more common to professional models (fabricated deck is a big one). Maybe open your search for reviews to something like "snapper ztr", in case you are including the model number.
One great thing is B&S also owns Ferris, and when looking at Snapper mowers...many new ones are heavily influenced by the Ferris mowers (suspensions on some, etc).
I have not owned a Snapper ZTR. Most reviews found will probably be at the commercial level of equipment. No bad reviews is actually better (in my view) than no good reviews. Often times people won't go online just to say how much they like something...but if they don't like it you can bet they'll put it out there.