As someone who invested a ridiculous amount of time trying to get x10 running, I would strongly argue against it being usable ;p
It's been around for a while, and sucked even when it was new. It can't cope with all the transformers and other "signal sinks" found plugged in all over the place in any home build in the last few decades. You end up with a whole hodgepodge of bridges and filters and custom run "neutral" wires.
When you do manage to get signals moving around your house, you then hit up against the second big problem. The x10 protocol is insanely primitive, and random noise will often also be a valid command. Nothing like the UPS plugged in upstairs turning your bedroom lights on every few hours..
I invested a fair bit of time and money into this (had an ocelot controller and I estimate about 20 billion dollars in filters and different varieties of inline switches and appliance modules that all sucked in their own unique way) before throwing it all in a box and moving forward with my life.
I still drag out my x10 gear around Christmas time, but that's about it.