For the most part - it works very well! However, occasionally, everything stops working. I blame the neighbors, the weather, the technology, and generally everything I can think of that might weaken or interfere with the signals. I even bought an ELK ESM1 X10 Signal Strength Meter to try to track down the signal and see how strong it was. Until today, I never determined what the pattern was - and therefore never completely solved it.
The breakthrough came today as I was trying once again to find out why the porch lights had not turned on automatically. I used the control pad that came with the Firecracker to turn them on and off manually, and still they refused to operate. Thinking that perhaps I should switch to a different house code to avoid potential interference, I turned one porch light switch to house code "I" - and it still did not turn on and off! I used the ELK meter to check outlets around the house and I discovered there was NO measurable signal. AHA! Perhaps the TM751 was dead.
I pulled out the TN751 and plugged it back in. To my surprise, a weak signal appeared. Slowly the light dawned. I use the TM751 alone to control the other systems. There is nothing plugged into the built-in socket. If the TM751 develops poor contacts with the socket, there are no telltale signs. That is exactly what had happened, and I suspect it has been the cause of intermittent performance I had been experiencing. I took the TM751 out one more time, and cleaned the plug prongs with MG Chemicals Super Contact Cleaner. The paper towel I used showed black residue - corrosion! - when I was done. I plugged the module back in, and tested the signal strength with the ELK meter. Excellent! back over 1 volt everywhere in the house. And sure enough, everything has worked perfectly ever since.
When troubleshooting X10, don't overlook poor contacts between the modules and the wall outlets. With signal levels so low, even minor conductivity losses can cause the whole system to become unreliable, or fail completely.
Here is another interesting story from January, 2010. I recently bought several nice LED nightlights to replace the energy-wasting incandescents I had been using. I bought the new nightlights at Home Depot. They are AmerTac Model #71352 0.15 Watt (that compares well to the 4 Watt incandescent nightlight I replaced!).
I noticed after I installed them that some of my X10 modules stopped working. I tried unplugging them one by one and eventually discovered that when one of the new nightlights was plugged into a plug circuit that had X10 modules installed on it, the modules stopped working. When I unplugged the nightlight, the modules began to work again. This behavior was consistent.
The new nightlight was probably creating electrical "noise" that interfered with the X10 signals. I even discovered that moving the nightlight to one of the other plus in that circuit enabled the X10 modules to work again. The plug that seemed to cause the interference is an ordinary plug, not a GFCI. The moral of the story is: If your trusty X10 modules suddenly stop working after you add some new electric device, no matter how small, try unplugging the device to see if the problem goes away. Mine did!