I did the upgrade this past weekend. I will say the new installer is great. The only issue I had with it is that we had the old KND module installed, so when the installer got to the point where you can choose which modules to install/upgrade, it errored with a message saying to contact Kaseya Support for that module to have them remove something from our AD environment (I don't recall the exact wording). So I started a ticket and pushed off actually doing the upgrade from Friday night to Saturday night, however Kaseya Support responded and said all I had to do was uninstall that module. I did that, and the rest of the upgrade went fine. It would have been nice if the installer had just told me to do that instead of explicitly saying to get Support involved.
Other than some minor issues with VNC on a few of our tech's PCs, and a handful of agents that won't upgrade to the latest version for some reason, everything went well. With a total of about 4,000 agents deployed, it can be a major productivity issue if our Kaseya system is down, so it was nice to have this go as smoothly as it did.