When I purchased my ISY I went ahead and bought the ISY994i ZW + Pro hoping to have as many hardware paths available for future expansion as possible. My next major purchase was an Insteon thermostat because as someone new to home automation with the Universal Devices ISY I wanted simplicity when connecting and working with all of my Insteon devices (wall switches, outlets, etc.).

So I am curious, do Zwave devices  work as smoothly as the Insteon devices do when using them with the ISY994i, especially since I have the ZWave module in my unit?
 

As long as you have enough devices to build a strong mesh network then you'll be fine with zwave devices. While performance varies (as with anything) it comes down to expectations and what you are trying to accomplish

lilyoyo1 is right. Zwave works best when you have enough Zwave devices to have an effective mesh network. If  you don't want to add (more) Zwave wall switches, then add a few plugin devices. On the average these costs between $ 25 and $ 35. All Zwave Plus devices (excluding those that are battery powered) repeat the Zwave signal throughout the network. And when you add a plug-in device you can also use it (for example) to control a lamp.

I have about a dozen z-wave devices and several dozen Insteon.  I've been very satisfied with the performance of the z-wave devices.  I use several of Aotec Multi-Sensor 6 and they have performed flawlessly.  I always use them powered.

Always check in this forum to make sure the particular z-wave device is compatible with ISY.  Some are not, especially the multi-channel devices.  the nice thing about z-wave is that there are products from a number of different manufactures.

I just installed the Honeywell z-wave enabled thermostat.  Inclusion was no problem and so far performance has been perfect.  Response is very fast.  I'll be installing a second one for my downstairs unit tomorrow.