So we also made the switch this season as well--though this has been an ongoing process--we have three S-series turf tracers and 1 X series turf tracer with fuel injection. If they all could be fuel injected I would do it--x series burns like .9-1 gal/hour. We don't have sulkies of any flavor though we are considering a pro slide for the X series. We run a combination of two and three man mowing crews--two man crew--2 48" S series, three man crew 48" S, 52" X and walker 48" GHS (which seldom gets used other than spring and fall). The cost savings is huge and productivity is actually faster--we do mostly medium residential and small/medium commercial. The way the work gets done flows better and there is less down time for the operators--everyone is expected to perform any and all operations--you finish mowing there is a trimmer with your name on it, or an edger, or gardening tools/sprayer, or blower depending on what the progress is for the other crew members. When we had the rider the rider lagged to avoid trimming etc--in effect we leveled the playing field.
There are also some huge business ramifications that benefit larger companies (anyone bigger than solo) and this is why Brickman is big into using walkbehinds. Don't get me wrong there is a place for the zero turn, but it is a much smaller place than it currently occupies in the market.