We switched over late in 2011--and we held onto the Z for a while after that, but it wasn't ever getting used so I sold it. We have 1 TTX 52" and 3 TT S 48" ( I may sell one). We were running two, two man crews and then shrunk to one three man crew--thus the extra mower. We have a cheap sulky for the TTX, but it only gets used at my house and on one other account.
Honestly if you are at all concerned about turf health and are the least bit environmentally sensitive (which is a huge marketing part of this) you need to make the switch. Your bottom line will thank you as well. If you have a portfolio of accounts with wide open spaces larger than 2 acres--then have a Z, but make sure you can keep it busy. If you have large condo associations--with high acreage but that is all broken up the walkbehinds will be a huge savings. If you do only residential and you have employees the walkbehinds are a step in having fewer account damages--ie collisions, turn circles, ruts, tears on hills, etc....
But for this to work well you need to buy nice walkbehinds that have some ergonomic improvement and that come with good power plant options. Exmark turf tracers with the ECS, Toro has a high end hydro float deck walkbehind with ergonomic controls, the hustler isn't bad--cut quality and hill stability were lacking next to the TTS/TTX. But do persuade your dealer to let you try these for at least a week.
If you are a solo operator--this might not be for you--this is particularly powerful for companies running crews with 3 or more members. This is the reason Brickman does this as well as many larger commercial maintenance companies.
Exmark if you are lurking on here--we need Kohler 21 EFI gasoline on the S series---you give the propane option, we want the gasser