A few days ago I replaced my C-5 that has been in service for (I'm estimating) 2+ years. Generally the C-5 has been very reliable. I was a little reluctant to upgrade since it really didn't seem necessary. I bought a C-7 when it was released. Other than powering it up to register and using it to test a couple of devices from time to time, it has sat dormant on my workbench.
Following the directions in the Hubitat migration video, I had no problems with the process. Some of the devices and automations were a little slow to respond. After a couple of days with everything having time to settle, I would have to say that pretty much everything is noticeably faster. Also looking at the routing table, the vast majority of my devices are now communicating directly with the C-8, including some in my detached garage. This in itself is pretty impressive since my hub is located in the basement of the house.
I would have to say that this upgrade has been easy, without problems/glitches and a great improvement for performance. I'm quite pleased!