Well, first a bit of history...
X10 was developed in 1975 a 17 year old in Scotland. It works by asigning each all of the devices in your home a home code (a-m) then each device gets a device code (1-8) You can run multiple house codes in your home but a device can not interact with another device that is not in the same house code.
Devices speak to each other by transmitting a (ONEWAY) message through your electrical lines to each other. So if you send a device A-1 on signal the light with house code "A" and device code "1" will turn on. But there is no status returned so you have no error correction or confirmation of transaction. The other problem is that with X10 you have about an 75 % chance that a tv, florecent light or some other device plugged in to your home power will either not work or turn on/off when not wanted.
Now with Insteon, every additional device you add makes the system more reliable. All traffic is 2 way so if you turn a device on it will report that it did indeed turn on. If it didnt the signal will be sent again. Also the Insteon addresses are a 6 digit alpha numeric. Example: A1BE15
This removes the limitation of how many devices you can have in your home.
So to summorize, X10 = headache and frustration
Insteon = 100% satisfaction!
CM