OK so I did the upgrade. Took forever since I guess I have a lot of applications. So far most things seem to work fine. A few issues:
The installer failed 6 times to create a USB install drive. It would fail in random places. I think though this could be a hardware issue as I've seen flaky behavior from most USB drives on my system. After the 6th time I gave up and did a direct upgrade.
XP Mode is not supported. I relied on XP Mode to test some compatibility for applications that I develop. While Windows Pro 8 has Hyper-V, I can't enable it since my computer doesn't support SLAT (Intel DX48BT2, Core 2 Quad CPU Q9450). Bummer. I guess I can live without it since it's not that often that I still require it. A warning would have been nice though since I clearly had it set up on my system. This was is not listed anywhere in the Upgrade Assistant report.
I have not tested all of my music applications, but those I tested seem to work fine. One of my plugins didn't make it so I need to investigate further. Hopefully there is an updated version, or maybe a re-install will fix it.
In general things work well and I like the slickness of Windows 8. I have to say though I'm not a fan of the new blocky look in desktop mode. I'm sure I just need time to have it grow on me, but then again I don't think I'll ever get used to the fugly VS 2012.
Installing the WP8 SDK right now. If it all works fine I guess the upgrade would have been worth it.