Of course I have to jump in on this one...
I have been using X10 for years. I agree 100% that it is obsolete technology, and very low end compared to ZWave... However, they are dirt cheap, plentiful - and if installed correctly (with a properly installed phase coupler) can also offer 100% reliability. I have never had an X10 signal not get to its target device, and I've never had a device receive a stray signal not intended for it.
Latency kind of sucks - anywhere from about a quarter of a second to even a full second (but usually more around half a second). But this is due to the slow bus speed. However, I'm wrapping up some changes to the CM11A driver which greatly improve performance while dimming (a known cause of X10 bus saturation) by adding some of the documented direct dim modes to eliminate sending up to 20+ dim commands while changing lighting levels.
I'll try to make up a video here soon showing my X10 setup, showing the types of things I'm using it for (lighting control, HVAC control, doorbell, motion detection, door/window open/close status indication, etc.)
In the end, I always recommend letting your budget decide what automation system you use. Of course, if you can afford ZWave or Insteon, you would be stupid not to use them - they are great and proven products. But for us poor schmucks that can't afford to lay down a few thousand dollars for a luxury "toy" system, X10 is a great solution that lets us enjoy the same luxuries that other wealthier people enjoy. Linux in general gives us this power of choice to choose what fits us, and LinuxMCE has continued this tradition nicely, so it would also be stupid not to take advange of this added benefit if you can't afford the more expensive toys.
Just my thoughts