I was in the exact same boat as you, I have 40 Insteon devices, but I hated the Insteon door locks and thermostats.
I have had a great experience with the ISY and Z-wave.
I have two Kwikset 914 door locks, two Honeywell z-wave thermostats, and 2 aeon smart power plugs, all z-wave. Ironically, I feel like the z-wave devices respond more quickly and have better signal reliability than my Insteon devices.
I've never had any communication issues with any of the Z-wave devices.
All devices were setup quickly and without any problems.