I find the comments on this product interesting. They range from simple installation to it doesn't work. First of all, this is for a single pole switch, which means the light that the regular switch controlled had only one switch that turned it off and on. Single pole switches normally only have two wires coming to them plus a bare wire for a ground. In order for the light in a room to operate, it needs the power wire and the neutral wire connected to it. A switch breaks the power wire to the light. There are two wires coming into the switch, one wire brings the power to the switch and the other wire takes the power from the switch to the light. To make a general statement that wiring this timer is simple and done in 10 minutes is very misleading. Yes, some houses have been wired by bringing the power wire and the neutral wire into the switch box, splitting the power wire and letting the neutral wire run through the electric box behind the switch and on to the light. This is rare for it takes more wire to do wiring this way. Still there is not a red wire and this makes the wiring instructions almost worthless. One has to understand house wiring in order to wire this timer. If there are only two wires (insulated wires) in your switch box (don't count the completely bare copper ground wire), then you will need to have the neutral wire brought to the switch and this can be very time consuming and costly if you are paying an electrician to run the needed neutral wire.
In the majority of the situations, installing this timer is not a simple operation. Even if the correct wires are present, one needs to know which one is the power wire, the neutral wire, and the wire caring the power from the switch to the light. I believe that the comments that said this timer does not work, do not have it wired correctly. The instructions are not at all helpful to anyone who doesn't know anything about house wiring.
Definitely make sure you are installing this where a single pole switch was used. A three-way switch normally has a red wire in the box, but it is not the red wire that needs to be connected to this switch. You will have a three-way switch if two switches can turn the light off and on. One usually finds them at the entrance to a room and the exit from a room if the entrance and exit are at different locations.