There are a few reasons why we love smart thermostats:
1) You can control multiple zones of heating & cooling without walking around the house adjusting individual thermostats.
2) You can remotely access and control your house heating and cooling system via a smartphone or tablet.
3) We can integrate the HVAC into the other house systems so that the system does not waste energy when you are not home. (It knows when the alarm is on.)
We have been using the Crestron HVAC controls as our primary solution for the past several years because they allow the thermostats to be located out of sight in a mechanical area. Each thermostat connects to a small, paint-able temperature sensor, so we can eliminate unsightly thermostats in areas where aesthetics matter. The Crestron products allow us to group multiple sensors or thermostats so that we can control any number of thermostats using one interface.
The new Lutron QS system includes thermostats that work with their lighting control and motorized window shading systems. They have local and remotely located thermostats and their products also allow us to use wireless temperature sensors that can control any heating/air conditioning system.
Another product for HVAC control is the new Nest thermostat. These great little thermostats are inexpensive, connect to existing thermostat wiring, and connect to the WiFi network in the home. They have a simple online interface for remote control via PC, smartphone or tablet. They also have a learning feature that allows them to learn from your heating/cooling use and anticipate your needs. Although they do not work with remote temperature sensors and they cannot be grouped into zones yet, they are a very cool product at a reasonable price point.