Installing the adapter (I had one from my first installation process) was made more difficult because my zone control board does not have the nice labels that correspond to the labels on the power adapters. Hours of research later, I discovered the correct ports to use. As I only have one adapter, I chose the thermostat with the most issues--the one that I just installed in the boys' room. It took about an hour (and more research on the internet) because, despite following the directions, I kept seeing that the thermostat was not receiving power. It turns out that whoever initially installed the thermostats put the white wires into the red wire's ports on the control board and vice versa. This wasn't a problem when the system only had two wires, but the third wire added by the adapter meant that power could only flow in one direction. I fixed that ridiculous error, and the thermostat seems to be working fine.
The issue with Google charging for the adapters despite their instruction booklets saying that the adapters were free has made it so that we only have one more adapter on the way. We now have two of the more expensive "learning" thermostats because they can use remote sensors to regulate the temperature. As two of our zones have their thermostats in areas that are heated by other zones, we need them to run off the information from parts of the rooms other than where the thermostats are located (which was one of the main reasons I bought these new thermostats to begin with). Ironically, the more expensive thermostats use less power than the less expensive ones (like the one in the boys' room), so we are holding off on getting the power adapters for them for the time being.
We also decided to return one of the thermostats that would have required an adapter. Originally, we were just going to exchange it because it was clear that we got one that had been returned after it had been opened (and possibly used). However, since it was for a room that already had a programmable (if not app controlled) thermostat, and due to the extra expense of the adapter, I figured that it wasn't worth the bother.
We've already noticed improvements in how our house is heating. I hope that it turns out to be worth the effort.