I had an installation of about 20+ X10 devices in my prior house and used HomeSeer to control them. When I moved almost two years ago, I knew I wanted to stick with HomeSeer, but I wanted something more robust than X10. After much effort and trouble-shooting, my X10 was fairly reliable, but not immune to random lights on/off or missed commands.
I chose Insteon, which was truly an infant at the time. I now have about 21 months with Insteon and it has been very robust and reliable (and quite fast). Since I moved, I didn't need to integrate it with a legacy X10 system, but I would have probably tried that in my old house.
As Mike suggested, I tried a few switches just to test it out at first. It went pretty smoothly in terms of the Insteon side (HomeSeer's support of Insteon was in beta earlier on, but is now quite stable, in my experience). I now have a total of about 16 devices that are truly reliable. I'm actually looking to expand my Insteon cababilities now.
Again, it has matured nicely. I had been waiting for direct IR support, which is finally here. Other solutions have also built up over the last two years, including more switch and adapter choices.
Clearly, this isn't a comparative vote, since I didn't try the others, but my experience for Insteon has been positive and a pleasant upgrade over X10.