This thing is really cool. I wanted something that I could use to put my porch light on a timer with a CFL bulb, and this works perfectly.
Some other available products (light sensors and other timers, including a Honeywell timer similar to this one) only work with incandescent bulbs, which defeats the purpose of putting lights on a timer to save energy. I could leave my 26-watt CFL (100w equivalent) on all day and still use less electricity than a 100-watt incandescent on an 8-hour timer.
With this model, you can program either a single on/off cycle to repeat every day, or up to 7 separate on/off cycles for each individual day of the week. I think you can also set an every-day cycle and combine it with 6 additional day-specific cycles if you choose. The cover for the programming buttons also functions as the manual on/off button, and using it does not interfere with your programs.
There is another model that works with CFLs -and- can be set to vary its on/off times to match your local daylight calendar (Honeywell Econoswitch RPLS740B 7-Day Solar Time Table Programmable Switch) but for the extra cost, I didn't need that feature.
I had some trouble figuring out that you have to slide a toggle under the button cover to "ON" before the switch itself will work AT ALL. I thought that was a bit confusing in the instructions, so now you'll know. Programming is not very difficult but it took me a few tries to figure it out. When you are finished programming, just close the cover and press it once (as if you're turning the light on) to exit programming mode.
To use this timer, you need to have a light switch where the power enters at the switch rather than at the light fixture. That's because the timer itself needs to have electricity constantly, in order to operate while the light is turned off. If your power enters at the light fixture instead, shutting off the light would also kill the power to the timer. Most houses are wired the correct way, which typically means at least 1 pair of black/white wires entering the switch box. If instead there is no white wire at the switch, only two black wires or a black and a red, you won't be able to use this timer.
As a side note, this switch has some sort of capacitor inside that saves your programs during power outages. It isn't a battery, so you would never need to replace it. It has worked fine through a couple of brief outages so far.
Edit: Lost power for over a week from some nasty storms this summer (July 2012), but the timer remembered all of my programs and the clock was still correct. Awesome!