The biggest problem I had with X10 was reliability issues. Sometimes it would work other times not. After looking at different solutions including purchasing higher quaility X10 I went with Zwave.
I like the fact there is virtually no delay when pressing all lights off. All 15 modules turn off.
In term of Reliability I had to replace one switch in just shy of a year. This was a known defect that has since been fixed. This was covered by a 2 year warranty.
I really like that ELK is going to support it in the M1. I'm going to move everything over to the M1 and let Homeseer do mostly neat/show off tasks.
Cost is still cheaper then some others. Almost half of UPB
But I will stick with Zwave as they keep releasing products slowly.
Hopefully some others will chime in on Zwave, someone that isn't a vendor.