I upgraded to 8.1 last night, with very few issues.
Mailenable broke after the install and reboot.
The MTA Server Service would crash when trying to restart. I had to download the lastest Mailenable and re-install it
http://forum.swsoft.com/showthread.php?threadid=38672
My horde install is not working either (althought I cant say that it ever was. I can get to the login screen, but it ALLWAYS brings me back to the login screen no matter who I login as. What wierd is if I login with an invalid login, I get "Invalid Login" If I login with a valid login, I get brought back to the login screen anyway. Not to sure whats causing that, but I use HMailServer as my primary mailserver because it has free IMAP support, so it dont bug me anyway
The licence update from the control panel after the upgrade worked without issue, so no problems there.
One final note I guess. I was updating from 7.5.6 to 8.1. From the looks of things, you must be on 7.5.6 to go to 8.1. Prior versions of 7.5.x must be updated to 7.5.6 to be able to get to 8.1 properly.
Z
p.s. This was my windows box